Clarify which dictionaries are updateable
by using the wording from the Py2.7 docs.
merge
Fix #17967 - Fix related to regression on Windows.

os.path.join(*self.dirs) produces an invalid path on windows.
ftp paths are always forward-slash seperated like this. /pub/dir.
merge from 3.3

Fix #17967 - Fix related to regression on Windows.

os.path.join(*self.dirs) produces an invalid path on windows.
ftp paths are always forward-slash seperated like this. /pub/dir.