make os.path identical in Python 2 and 3 (#1459)

Part of #1427. I don't think we can actually merge these until we merge
os/__init__.pyi too, which will take a few more PRs.

Author: JelleZijlstra

stdlib/2/os/path.pyi

@@ -10,7 +10,12 @@ from typing import (
 )
 
 _T = TypeVar('_T')
-_PathType = Union[bytes, Text]
+
+if sys.version_info >= (3, 6):
+    from builtins import _PathLike
+    _PathType = Union[bytes, Text, _PathLike]
+else:
+    _PathType = Union[bytes, Text]
 
 # ----- os.path variables -----
 supports_unicode_filenames = False
@@ -55,20 +60,23 @@ def isdir(path: _PathType) -> bool: ...
 def islink(path: _PathType) -> bool: ...
 def ismount(path: _PathType) -> bool: ...
 
-# Make sure signatures are disjunct, and allow combinations of bytes and unicode.
-# (Since Python 2 allows that, too)
-# Note that e.g. os.path.join("a", "b", "c", "d", u"e") will still result in
-# a type error.
-@overload
-def join(__p1: bytes, *p: bytes) -> bytes: ...
-@overload
-def join(__p1: Text, *p: _PathType) -> Text: ...
-@overload
-def join(__p1: bytes, __p2: Text, *p: _PathType) -> Text: ...
-@overload
-def join(__p1: bytes, __p2: bytes, __p3: Text, *p: _PathType) -> Text: ...
-@overload
-def join(__p1: bytes, __p2: bytes, __p3: bytes, __p4: Text, *p: _PathType) -> Text: ...
+if sys.version_info < (3, 0):
+    # Make sure signatures are disjunct, and allow combinations of bytes and unicode.
+    # (Since Python 2 allows that, too)
+    # Note that e.g. os.path.join("a", "b", "c", "d", u"e") will still result in
+    # a type error.
+    @overload
+    def join(__p1: bytes, *p: bytes) -> bytes: ...
+    @overload
+    def join(__p1: Text, *p: _PathType) -> Text: ...
+    @overload
+    def join(__p1: bytes, __p2: Text, *p: _PathType) -> Text: ...
+    @overload
+    def join(__p1: bytes, __p2: bytes, __p3: Text, *p: _PathType) -> Text: ...
+    @overload
+    def join(__p1: bytes, __p2: bytes, __p3: bytes, __p4: Text, *p: _PathType) -> Text: ...
+else:
+    def join(path: AnyStr, *paths: AnyStr) -> AnyStr: ...
 
 def normcase(path: AnyStr) -> AnyStr: ...
 def normpath(path: AnyStr) -> AnyStr: ...

stdlib/3/os/path.pyi

@@ -60,15 +60,31 @@ def isdir(path: _PathType) -> bool: ...
 def islink(path: _PathType) -> bool: ...
 def ismount(path: _PathType) -> bool: ...
 
-def join(path: AnyStr, *paths: AnyStr) -> AnyStr: ...
+if sys.version_info < (3, 0):
+    # Make sure signatures are disjunct, and allow combinations of bytes and unicode.
+    # (Since Python 2 allows that, too)
+    # Note that e.g. os.path.join("a", "b", "c", "d", u"e") will still result in
+    # a type error.
+    @overload
+    def join(__p1: bytes, *p: bytes) -> bytes: ...
+    @overload
+    def join(__p1: Text, *p: _PathType) -> Text: ...
+    @overload
+    def join(__p1: bytes, __p2: Text, *p: _PathType) -> Text: ...
+    @overload
+    def join(__p1: bytes, __p2: bytes, __p3: Text, *p: _PathType) -> Text: ...
+    @overload
+    def join(__p1: bytes, __p2: bytes, __p3: bytes, __p4: Text, *p: _PathType) -> Text: ...
+else:
+    def join(path: AnyStr, *paths: AnyStr) -> AnyStr: ...
 
 def normcase(path: AnyStr) -> AnyStr: ...
 def normpath(path: AnyStr) -> AnyStr: ...
 if sys.platform == 'win32':
     def realpath(path: AnyStr) -> AnyStr: ...
 else:
     def realpath(filename: AnyStr) -> AnyStr: ...
-def relpath(path: AnyStr, start: AnyStr = ...) -> AnyStr: ...
+def relpath(path: AnyStr, start: _PathType = ...) -> AnyStr: ...
 
 def samefile(path1: _PathType, path2: _PathType) -> bool: ...
 def sameopenfile(fp1: int, fp2: int) -> bool: ...