fix types in os.path (#1363)

In Python 2, doing
    os.path.join(u"foo", "bar")
is actually legal, and returns a unicode string.
Also os.path.relpath always returns the type of its first argument.

(The solution is not perfect -- e.g.
    os.path.join("a", "b", "c", "d", u"e")
will still result in a type error. )

stdlib/2/os/path.pyi

@@ -55,15 +55,28 @@ def isdir(path: _PathType) -> bool: ...
 def islink(path: _PathType) -> bool: ...
 def ismount(path: _PathType) -> bool: ...
 
-def join(path: AnyStr, *paths: AnyStr) -> AnyStr: ...
+# 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: ...
 
 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: ...