py3: make scmposix.userrcpath() return bytes

We are making sure that we deal with bytes as much we can. This is a part
of fixing functions so that they return bytes if they have to.
Used encoding.environ to return bytes.

After this patch, scmposix.userrcpath() returns bytes and scmutil.osrcpath()
will also return bytes if the platform is posix. Functions is scmposix returns
bytes on Python 3 now.

mercurial/scmposix.py

@@ -4,6 +4,7 b' import os'
 import sys
 
 from . import (
+    encoding,
     osutil,
 )
 
@@ -34,6 +35,6 b' def systemrcpath():'
 
 def userrcpath():
     if sys.platform == 'plan9':
-        return [os.environ['home'] + '/lib/hgrc']
+        return [encoding.environ['home'] + '/lib/hgrc']
     else:
         return [os.path.expanduser('~/.hgrc')]