python.lang: make Python 3 the default

Python 2 has been EOL'd for quite some time. So we want "Python" to just mean "Python 3" at this point.

This also alters the default for the user in a subtle way because the list uses g_utf8_collate() to sort, meaning "Python" sorts before "Python 2". Since they have the same mime-types and globs, what was Python 3 will now sort first.

Fixes #137 (closed)