wireprotov2peer: rewrite character traversal to use slices
Otherwise on Python 3 we iterate over integers instead of a
bytes instance and the comparison fails.
Differential Revision: https://phab.mercurial-scm.org/D5698
wireprotov2server: don't attempt to cast command name
I'm not sure why this was added. The command name should already
be bytes.
Differential Revision: https://phab.mercurial-scm.org/D5697