Add support for GraalPy

[timfel]

There are some checks for PyPy, but the branches for PyPy are really related to GC differences to CPython which apply to GraalPy just the same. With these changes I can pass most of the test suite on GraalPy (modulo some bugs on our end)

[Zac-HD]

Hi Tim - thanks for this contribution! I've admired Graal from afar for some years now, and I'm really excited to get Hypothesis running on top of it 😁

It looks like the remaining CI failures are down to (1) a missing hypothesis-python/RELEASE.rst file, where we'll need to describe a patch release to get this through our CD system, and (2) a mysterious hang in our coverage jobs - I noticed this in #3589 too, so I don't think it's anything to do with these changes. Please bear with me if this takes a while to track down, I'm pretty short on time at the moment.

[timfel]

(1) a missing hypothesis-python/RELEASE.rst file, where we'll need to describe a patch release to get this through our CD system

I looked at that, but was unsure what the file should look like, but I just found an example in a different PR, so I'll add something like it.

(2) a mysterious hang in our coverage jobs - I noticed this in #3589 too, so I don't think it's anything to do with these changes.

Thank you for confirming that - I was stumped how I might have caused that :)

[Zac-HD]

LGTM - thanks again, Tim!

I'll merge as soon as we debug and fix this hang 🙂

[Zac-HD]

Cherry-picked into #3581 and released! (although the docs build is failing so it'll be a few hours before this appears in the changelog)

Thanks again Tim!