extension: access special module members using sysstr These extensions variables and mapping are module attributes so they should be dealt with unicode `str` in Python 3. We move the part that deal with reading theses variable and checking their validity to use unicode `str` string.

sshpeer: convert command name to sysstr before accessing method Method name are `str` so we should pass them as `str`.

localrepo: purge filecache attribute using there unicode name This could be better, but that's a good step.