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 @@
-import os  # @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
+
+import os
+import sys
 from typing import Union, Any
 
 
 TBD = Any
-PathLike = Union[str, 'os.PathLike[str]']
+
+if sys.version_info[:2] < (3, 6):
+    # os.PathLike (PEP-519) only got introduced with Python 3.6
+    PathLike = str
+elif sys.version_info[:2] < (3, 9):
+    # Python >= 3.6, < 3.9
+    PathLike = Union[str, os.PathLike]
+elif sys.version_info[:2] >= (3, 9):
+    # os.PathLike only becomes subscriptable from Python 3.9 onwards
+    PathLike = Union[str, os.PathLike[str]]