Summary: Also make it compatible with Windows by making it a no-op on Windows.
Test Plan:
`make local` and try from IPython:
```
In [1]: from hgext import patchrmdir
In [2]: os.rmdir
Out[2]: <function posix.rmdir>
In [3]: patchrmdir.uisetup(None)
In [4]: os.rmdir
Out[4]: <functools.partial at 0x375f580>
```
Reviewers: durham, #mercurial
Reviewed By: durham
Subscribers: durham, fried
Differential Revision: https://phabricator.intern.facebook.com/D6681125
Signature: 6681125:1515523144:e186b45978d4a82ec8a0d18a02b0a524fca1c3b3
