Add a python blueprint

Things we have to consider are if or how we would want to handle pip packages. We could also decide to use the system python only for bootstrapping and to always use this python.

This blueprint lacks support for Tk and mingw. Relocatability issues will only show up once we cached it, but should be easily fixable.

If required, we can add .pc files on Windows.

@vkrause @jlskuz @bcooksley @tcanabrava @sgerlach