solves Line2D attribute error with matplotlib 3.7.0 (#3) [copy of nschloe#571]

* solves attribute error with matplotlib `3.7.0`

the error on most recent matplotlib version is the following:
```bash
AttributeError: 'Line2D' object has no attribute '_us_dashSeq'
```

* Update src/tikzplotlib/_path.py

---------

Co-authored-by: Thomas Gillis <[email protected]>

Author: st--

src/tikzplotlib/_path.py

@@ -469,8 +469,12 @@ def mpl_linestyle2pgfplots_linestyle(data, line_style, line=None):
         default_dashOffset, default_dashSeq = mpl.lines._get_dash_pattern(line_style)
 
         # get dash format of line under test
-        dashSeq = line._us_dashSeq
-        dashOffset = line._us_dashOffset
+        try:
+            dashOffset, dashSeq = line._dash_pattern[:2]
+        except AttributeError:
+            # backwards-compatibility with matplotlib < 3.7.0
+            dashOffset = line._us_dashOffset
+            dashSeq = line._us_dashSeq```
 
         lst = list()
         if dashSeq != default_dashSeq: