fix(types): get the os.PathLike type as correctly as possible

This should make our internal PathLike type compatible with Python <
3.6 and < 3.9.

Author: muggenhor

git/types.py

@@ -1,6 +1,20 @@
-importos# @UnusedImport ## not really unused, is in type string
+# -*- coding: utf-8 -*-
+# This module is part of GitPython and is released under
+# the BSD License: http://www.opensource.org/licenses/bsd-license.php
+
+importos
+importsys
 fromtypingimportUnion, Any
 
 
 TBD=Any
-PathLike=Union[str, 'os.PathLike[str]']
+
+ifsys.version_info[:2] < (3, 6):
+# os.PathLike (PEP-519) only got introduced with Python 3.6
+PathLike=str
+elifsys.version_info[:2] < (3, 9):
+# Python >= 3.6, < 3.9
+PathLike=Union[str, os.PathLike]
+elifsys.version_info[:2] >= (3, 9):
+# os.PathLike only becomes subscriptable from Python 3.9 onwards
+PathLike=Union[str, os.PathLike[str]]