Update os.path module to use file descriptor where possible

Fix #1653
Use `_FdOrPathType` when possible in `os.path` module.

Author: Mateusz Kurek

stdlib/3/os/__init__.pyi

@@ -183,10 +183,7 @@ if sys.version_info >= (3, 6):
     from builtins import _PathLike as PathLike  # See comment in builtins
 
 _PathType = path._PathType
-if sys.version_info >= (3, 3):
-    _FdOrPathType = Union[int, _PathType]
-else:
-    _FdOrPathType = _PathType
+_FdOrPathType = path._FdOrPathType
 
 if sys.version_info >= (3, 6):
     class DirEntry(PathLike[AnyStr]):

stdlib/3/os/path.pyi

@@ -17,6 +17,11 @@ if sys.version_info >= (3, 6):
 else:
     _PathType = Union[bytes, Text]
 
+if sys.version_info >= (3, 3):
+    _FdOrPathType = Union[int, _PathType]
+else:
+    _FdOrPathType = _PathType
+
 # ----- os.path variables -----
 supports_unicode_filenames = False
 # aliases (also in os)
@@ -42,21 +47,21 @@ if sys.version_info >= (3, 5):
 def commonprefix(list: Sequence[AnyStr]) -> Any: ...
 
 def dirname(path: AnyStr) -> AnyStr: ...
-def exists(path: _PathType) -> bool: ...
+def exists(path: _FdOrPathType) -> bool: ...
 def lexists(path: _PathType) -> bool: ...
 def expanduser(path: AnyStr) -> AnyStr: ...
 def expandvars(path: AnyStr) -> AnyStr: ...
 
 # These return float if os.stat_float_times() == True,
 # but int is a subclass of float.
-def getatime(path: _PathType) -> float: ...
-def getmtime(path: _PathType) -> float: ...
-def getctime(path: _PathType) -> float: ...
+def getatime(path: _FdOrPathType) -> float: ...
+def getmtime(path: _FdOrPathType) -> float: ...
+def getctime(path: _FdOrPathType) -> float: ...
 
-def getsize(path: _PathType) -> int: ...
+def getsize(path: _FdOrPathType) -> int: ...
 def isabs(path: _PathType) -> bool: ...
-def isfile(path: _PathType) -> bool: ...
-def isdir(path: _PathType) -> bool: ...
+def isfile(path: _FdOrPathType) -> bool: ...
+def isdir(path: _FdOrPathType) -> bool: ...
 def islink(path: _PathType) -> bool: ...
 def ismount(path: _PathType) -> bool: ...
 
@@ -86,7 +91,7 @@ else:
     def realpath(filename: AnyStr) -> AnyStr: ...
 def relpath(path: AnyStr, start: _PathType = ...) -> AnyStr: ...
 
-def samefile(path1: _PathType, path2: _PathType) -> bool: ...
+def samefile(path1: _FdOrPathType, path2: _FdOrPathType) -> bool: ...
 def sameopenfile(fp1: int, fp2: int) -> bool: ...
 # TODO
 # def samestat(stat1: stat_result,