Clarify Python version support

[Rosuav]

https://pagekite.net/wiki/OpenSource/ says "a modern Python (2.x) interpreter", which is now a contradiction in terms. From other issues here I gather that there is some sort of Python 3 support, but which versions of Python 3 are supported? Normally I'd go look at PyPI and the metadata there, but pagekite doesn't seem to be listed.