Ensure backwards compatibility with Python 2

Use python (which often defaults to python 2.x) instead of forcing python2
Also make it compatible with python 2.7 and 3.4

Tested with:
$ python2.7 libpurple/purple-url-handler xmpp:foo@example.com
$ python3.4 libpurple/purple-url-handler xmpp:bar@example.com
(grafted from a6f8c7cbdb24eb7113268beede61db120f940917)