Pin cython to below version 3, Python 3.11 support

[kvelicka]

Cython 3 includes backwards incompatible changes so it's no longer possible to install pycapnp from source.

I'm not sure if that tox file change is enough to start running CI for 3.11, but I'm much less worried about that part than the broken builds.

Fixes #319

[haata]

tox.ini isn't used within GitHub Actions so I believe we already have Python 3.11 support.

v1.3.0 already has python 3.11 wheels.
https://pypi.org/project/pycapnp/#files

But your fix is appreciated regardless 👍

[kvelicka]

Yep apologies for ignorance, I'm not quite up to date with the myriad of ways in which python packaging/github actions are configured 😅

[alex-reilly-pronto]

Is there any way to fix this for older versions as well? We're running 1.1.0

[kvelicka]

Would it be possible to release a new version with these pins on PyPI?

[fungs]

I'm running into this issue with a project on Python 3.12. Pip install fails as there is no prebuilt wheel for it, I assume. For now, I got around it using the cython constraint workaround in #319 and bundling upstream capnproto v1.0.2 into the package build.

However, the easier way to fix the for my development stage now was to install the beta like pip install pycapnp==2.0.0b2.

[haata]

I'll try to push the 2.0.0 in the next few days.