Add pytz (Python) implementation

[mattjohnsonpint]

Implement tzvalidate using pytz library for Python.

Also, note that another common tzdb library for Python is dateutil, but dateutil uses zic-compiled data, while pytz ships its own copy.

[jskeet]

Does this mean a PR is coming? ;) (I'm happy to give it a go, but if you're already looking, I'll wait instead...)

[mattjohnsonpint]

Thought I would add some to the backlog, but I'm not too picky on who does them. I may have cycles in a few weeks. Feel free to beat me to it. :)

[jskeet]

pytz is currently shipping 2015d as far as I can tell... do you know if there's an easy way to run its zic-equivalent code? Looking at the code in launchpad.net, I can only see code to read in the zic-compiled data.