Looking at the Cython code I spotted a few areas where it contains special paths for Pyston v1.
Pyston v1 only supports python2 and is dead but we did release a new Pyston v2 which is a fork of CPython 3.8.
As we don't currently set `PYSTON_VERSION` in v2 the current code can't trigger and can be removed.

I hoped we would not need any special casing for v2 because it should be fully compatible with CPython but
https://github.com/cython/cython/issues/4200 may temporarily need one but I think the current
code is overkill and if necessary I can add (part of) it back later on.