py3: introduce and use pycompat.getargspec
This is getfullargspec on py3, which means we can't use namedtuple
named accessors for all fields (eg keywords from getargspec is varkw
from getfullargspec, with the same meaning). Solves some warning
issues on Python 3.
I didn't clean up httpclient because that's vendored code I think we
should probably discard, and I didn't touch the manpage generator for
now either.
Differential Revision: https://phab.mercurial-scm.org/D2251