PathLike cleanup (#5242)

Copy builtins._PathLike to os.PathLike

Use os.PathLike exclusively outside of builtins

stdlib/genericpath.pyi

@@ -1,5 +1,6 @@
 import sys
-from typing import AnyStr, Sequence, Text, Union
+from _typeshed import AnyPath
+from typing import AnyStr, Sequence, Text
 
 if sys.version_info >= (3, 0):
     def commonprefix(m: Sequence[str]) -> str: ...
@@ -7,13 +8,7 @@ if sys.version_info >= (3, 0):
 else:
     def commonprefix(m: Sequence[AnyStr]) -> AnyStr: ...
 
-if sys.version_info >= (3, 6):
-    from builtins import _PathLike
-    def exists(path: Union[AnyStr, _PathLike[AnyStr]]) -> bool: ...
-
-else:
-    def exists(path: Text) -> bool: ...
-
+def exists(path: AnyPath) -> bool: ...
 def isfile(path: Text) -> bool: ...
 def isdir(s: Text) -> bool: ...
 def getsize(filename: Text) -> int: ...