Support PyPy

The cpyext layer of PyPy has improved a lot over the years and some weeks ago pycairo passed all tests with it (some fixes are not in a pypy release yet). Excluding refcounting tests of course, due to GC design differences.

pycairo: https://github.com/pygobject/pycairo/issues/90

PyPy bugs:

• https://foss.heptapod.net/pypy/pypy/-/issues/1434
• https://foss.heptapod.net/pypy/pypy/-/issues/2780
• https://foss.heptapod.net/pypy/pypy/-/issues/2788
• https://foss.heptapod.net/pypy/pypy/-/issues/2792
• https://foss.heptapod.net/pypy/pypy/-/issues/2804
• https://foss.heptapod.net/pypy/pypy/-/issues/2806