force paths to str before joining in canconical_filename

```
================================== FAILURES ===================================
________________ UnicodeFilePathsTest.test_accented_directory _________________
[gw1] win32 -- Python 2.7.17 C:\projects\coveragepy\.tox\py27\Scripts\python.EXE
args = (<tests.test_process.UnicodeFilePathsTest testMethod=test_accented_directory>,)
kwargs = {}
    @functools.wraps(method)
    def _wrapper(*args, **kwargs):
        try:
>           result = method(*args, **kwargs)
tests\coveragetest.py:47:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
tests\test_process.py:1320: in test_accented_directory
    self.assertEqual(out, "accented\n")
E   AssertionError: 'Traceback (most recent call last):\n  File "C:\\projects\\coveragepy\\.tox\\py2 [truncated]... != 'accented\n'
E   + accented
E   - Traceback (most recent call last):
E   -   File "C:\projects\coveragepy\.tox\py27\Scripts\coverage-script.py", line 11, in <module>
E   -     load_entry_point('coverage', 'console_scripts', 'coverage')()
E   -   File "c:\projects\coveragepy\coverage\cmdline.py", line 865, in main
E   -     status = CoverageScript().command_line(argv)
E   -   File "c:\projects\coveragepy\coverage\cmdline.py", line 582, in command_line
E   -     return self.do_run(options, args)
E   -   File "c:\projects\coveragepy\coverage\cmdline.py", line 737, in do_run
E   -     self.coverage.start()
E   -   File "c:\projects\coveragepy\coverage\control.py", line 524, in start
E   -     self._init()
E   -   File "c:\projects\coveragepy\coverage\control.py", line 253, in _init
E   -     self._debug = DebugControl(self.config.debug, self._debug_file)
E   -   File "c:\projects\coveragepy\coverage\debug.py", line 36, in __init__
E   -     def __init__(self, options, output):
E   -   File "c:\projects\coveragepy\coverage\pytracer.py", line 135, in _trace
E   -     disp = self.should_trace(filename, frame)
E   -   File "c:\projects\coveragepy\coverage\control.py", line 315, in _should_trace
E   -     disp = self._inorout.should_trace(filename, frame)
E   -   File "c:\projects\coveragepy\coverage\inorout.py", line 309, in should_trace
E   -     reason = self.check_include_omit_etc(disp.source_filename, frame)
E   -   File "c:\projects\coveragepy\coverage\inorout.py", line 339, in check_include_omit_etc
E   -     if self.source_match.match(filename):
E   -   File "c:\projects\coveragepy\coverage\files.py", line 232, in match
E   -     fpath = self.aliases.map(fpath)
E   -   File "c:\projects\coveragepy\coverage\files.py", line 413, in map
E   -     new = canonical_filename(new)
E   -   File "<decorator-gen-2>", line 2, in canonical_filename
E   -   File "C:\projects\coveragepy\.tox\py27\lib\site-packages\contracts\main.py", line 270, in contracts_checker
E   -     result = function_(*args, **kwargs)
E   -   File "c:\projects\coveragepy\coverage\files.py", line 67, in canonical_filename
E   -     f = os.path.join(path, filename)
E   -   File "c:\python27\lib\ntpath.py", line 85, in join
E   -     result_path = result_path + p_path
E   - UnicodeDecodeError: 'ascii' codec can't decode byte 0xe2 in position 124: ordinal not in range(128)
E   - Coverage.py warning: Trace function changed, measurement is likely wrong: None (trace-changed)
---------------------------- Captured stdout call -----------------------------
Traceback (most recent call last):
  File "C:\projects\coveragepy\.tox\py27\Scripts\coverage-script.py", line 11, in <module>
    load_entry_point('coverage', 'console_scripts', 'coverage')()
  File "c:\projects\coveragepy\coverage\cmdline.py", line 865, in main
    status = CoverageScript().command_line(argv)
  File "c:\projects\coveragepy\coverage\cmdline.py", line 582, in command_line
    return self.do_run(options, args)
  File "c:\projects\coveragepy\coverage\cmdline.py", line 737, in do_run
    self.coverage.start()
  File "c:\projects\coveragepy\coverage\control.py", line 524, in start
    self._init()
  File "c:\projects\coveragepy\coverage\control.py", line 253, in _init
    self._debug = DebugControl(self.config.debug, self._debug_file)
  File "c:\projects\coveragepy\coverage\debug.py", line 36, in __init__
    def __init__(self, options, output):
  File "c:\projects\coveragepy\coverage\pytracer.py", line 135, in _trace
    disp = self.should_trace(filename, frame)
  File "c:\projects\coveragepy\coverage\control.py", line 315, in _should_trace
    disp = self._inorout.should_trace(filename, frame)
  File "c:\projects\coveragepy\coverage\inorout.py", line 309, in should_trace
    reason = self.check_include_omit_etc(disp.source_filename, frame)
  File "c:\projects\coveragepy\coverage\inorout.py", line 339, in check_include_omit_etc
    if self.source_match.match(filename):
  File "c:\projects\coveragepy\coverage\files.py", line 232, in match
    fpath = self.aliases.map(fpath)
  File "c:\projects\coveragepy\coverage\files.py", line 413, in map
    new = canonical_filename(new)
  File "<decorator-gen-2>", line 2, in canonical_filename
  File "C:\projects\coveragepy\.tox\py27\lib\site-packages\contracts\main.py", line 270, in contracts_checker
    result = function_(*args, **kwargs)
  File "c:\projects\coveragepy\coverage\files.py", line 67, in canonical_filename
    f = os.path.join(path, filename)
  File "c:\python27\lib\ntpath.py", line 85, in join
    result_path = result_path + p_path
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe2 in position 124: ordinal not in range(128)
Coverage.py warning: Trace function changed, measurement is likely wrong: None (trace-changed)
=========================== short test summary info ===========================
```

Author: graingert

coverage/files.py

@@ -64,12 +64,13 @@ def canonical_filename(filename):
             for path in [os.curdir] + sys.path:
                 if path is None:
                     continue
-                f = os.path.join(path, filename)
+
                 try:
-                    exists = os.path.exists(f)
+                    f = os.path.join(str_filename(path), str_filename(filename))
                 except UnicodeError:
-                    exists = False
-                if exists:
+                    continue
+
+                if os.path.exists(f):
                     cf = f
                     break
         cf = abs_file(cf)
@@ -143,6 +144,21 @@ def actual_path(filename):
         return filename
 
 
+if env.PY2:
+    def str_filename(filename):
+        return (
+            filename
+            if isinstance(filename, str)
+            else filename.encode(
+                sys.getfilesystemencoding() or sys.getdefaultencoding()
+            )
+        )
+else:
+    @contract(filename='unicode', returns='unicode')
+    def str_filename(filename):
+        return filename
+
+
 if env.PY2:
     @contract(returns='unicode')
     def unicode_filename(filename):