Clear any prior readline imports in unit test

Since the tests are run in Python's non-interactive mode we don't expect
readline to be imported. Unfortunately it manages to sneak in on the
latest Python. In Python 3.13.0 pdb now imports rlcompleter to standardise
its command-line completion (see python/cpython#112948). This is fine, but
pytest also imports pdb as part of its configuration as it wants to monkey
patch it in `pytest/src/_pytest/debugging.py`...

This causes readline to be imported before test.py is imported, thwarting
the `import_alternative_readline_module` function.

The solution is to clear any preloaded readline modules at the start
of the unit test by hacking it out of `sys.modules`.

Author: ludwigschwardt

tests/test.py

@@ -3,6 +3,12 @@
 
 import gnureadline
 
+# Since the tests are non-interactive we don't expect readline to be imported.
+# Unfortunately it manages to sneak in on Python 3.13.0 via pytest and pdb (see
+# python/cpython#112948 and pytest/src/_pytest/debugging.py). Enforce its absence.
+if 'readline' in sys.modules:
+    del sys.modules['readline']
+
 
 def import_alternative_readline_module():
     """This forcibly imports our alternative readline.py module over the standard one."""