urllibcompat: move some adapters from pycompat to urllibcompat
These are all the httpserver and urllib.* aliases. They seem to make
more sense in the slightly-more-specific urllibcompat package than the
general-purpose pycompat.
Differential Revision: https://phab.mercurial-scm.org/D935
urllibcompat: new library to help abstract out some python3 urllib2 stuff
Doing a new file instead of pycompat because I'm starting to feel like
pycompat is getting a little enormous in terms of scope.
Differential Revision: https://phab.mercurial-scm.org/D890