commitextras: fix on Python 3 by using sysstrs for __dict__ ops

I'm dubious of the __dict__ shenanigans in use here, but lack the
enthusiasm for figuring out why that was done right now.

# skip-blame just some r prefixes

Differential Revision: https://phab.mercurial-scm.org/D2435

hgext/commitextras.py

@@ -70,7 +70,7 b' def _commit(orig, ui, repo, *pats, **opt'
 
         # This __dict__ logic is needed because the normal
         # extension.wrapfunction doesn't seem to work.
-        repo.__dict__['commit'] = _wrappedcommit
+        repo.__dict__[r'commit'] = _wrappedcommit
         return orig(ui, repo, *pats, **opts)
     finally:
-        del repo.__dict__['commit']
+        del repo.__dict__[r'commit']