py3: use encoding.environ instead of os.environ

This complains while running hg version on Python 3.5

mercurial/scmutil.py

@@ -753,7 +753,7 b' def rcpath():'
     if no HGRCPATH, use default os-specific path.'''
     global _rcpath
     if _rcpath is None:
-        if 'HGRCPATH' in os.environ:
+        if 'HGRCPATH' in encoding.environ:
             _rcpath = []
             for p in os.environ['HGRCPATH'].split(os.pathsep):
                 if not p: