Add soft dependency on httplib2 0.9.0 for cacerts

Added the new version dependency only to setup.py
which ensures anyone packaging pywikibot provides
a version of httplib2 that has no known problems
with pywikibot.

Updated README for conversion:
- Remove mention of simplejson, as it is included
  in all supported versions of python.
- Remove note about unsupported httplib2 0.4.0.

Bug: 65189
Change-Id: I1917c463256443262b723ca1dead50308c5b585e
Merge "Add soft dependency on httplib2 0.9.0 for cacerts"