py3: convert opts keys to bytes before passing to core APIs

evolve: further clarify when update is performed

py3: avoid comparing int and None

py3: avoid "%r" for list of byte strings, which produces b'' on py3

py3: avoid "%r" for byte string, which produces b'' on py3 Replaced by "'%s'", which I think is clearer anyway.

py3: replace str(ctx) by bytes(ctx) These are all for messages to the user and we don't want unicode for that.

py3: use inspect.signature() instead of inspect.getargspec() on py3

py3: use array.array.{to,from}bytes() on py3 array.array.{to,from}string() still exists on py3, but they're deprecated and generate warnings. I've put the compat function in compat.pt for now. We can move into a dedicated pycompat.py if we end up with a lot of py3 compat stuff.

py3: config values can be bytes, but never unicode

py3: call branchmap.items() on py3 and continue to call iteritems() on py2 Mercurial's source transformer also replaces the 'def iteritems(' in branchmap by 'def items(', so we need to call whichever version is there.

py3: switch from iteritems() to items() in the topics extension The only remaining iteritems() call is on branchmap. That will be dealt with in the next patch.

py3: filter() now returns a generator, so wrap when we need a list