clarify python-xstatic/ github Organisation

[henning]

When googling for "python xstatic" the first gitub result i get is this:

https://github.com/python-xstatic/xstatic

It seems to be not updated anymore.

[ThomasWaldmann]

Looks like we forgot about that one and created a new one here at https://github.com/xstatic-py/xstatic/ .