Support python3

[dbarnett]

Maktaba has a little bit of python code in the python/ subdir for things like json and diff processing, and some helpers in maktaba#python# to help plugin developers load their own python cleanly. That python code isn't all python3 compatible.

We should update all of that to be py2/py3 compatible and get the tests running against py3 builds of vim so that it's possible to run maktaba plugins on new builds of vim that enable py3 instead of py2 (py2/py3 can't really coexist in the same vim instance, see http://vimdoc.sourceforge.net/htmldoc/if_pyth.html).

[dbarnett]

Just filed #194 and #195 to get rid of some of our incidental dependencies on python2.

[dbarnett]

Maktaba has no more hard dependencies on python2!

One qualifier on that: It has a python2 fallback for JSON support if native vim JSON support is missing. But (a) newer versions of vim likely to opt for +python3 will also tend to have native JSON support, and (b) that's not a hard dep, it will fall back to the vimscript implementation if the :python command is missing.