PathLike cleanup (#5242)

Copy builtins._PathLike to os.PathLike

Use os.PathLike exclusively outside of builtins

stdlib/builtins.pyi

@@ -57,7 +57,6 @@ from typing import (
     Union,
     ValuesView,
     overload,
-    runtime_checkable,
 )
 from typing_extensions import Literal, SupportsIndex
 
@@ -959,11 +958,10 @@ if sys.version_info >= (3, 7):
 def callable(__obj: object) -> bool: ...
 def chr(__i: int) -> str: ...
 
-# This class is to be exported as PathLike from os,
-# but we define it here as _PathLike to avoid import cycle issues.
+# We define this here instead of using os.PathLike to avoid import cycle issues.
 # See https://github.com/python/typeshed/pull/991#issuecomment-288160993
 _AnyStr_co = TypeVar("_AnyStr_co", str, bytes, covariant=True)
-@runtime_checkable
+
 class _PathLike(Protocol[_AnyStr_co]):
     def __fspath__(self) -> _AnyStr_co: ...