Pin cython to below version 3, Python 3.11 support (#320)

* Pin cython to below version 3

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

* Add py311 environment

I'm not sure if this is necessary, but 3.11 is out so might as well?

pyproject.toml

@@ -1,5 +1,5 @@
 [build-system]
-requires = ["setuptools", "wheel", "pkgconfig", "cython"]
+requires = ["setuptools", "wheel", "pkgconfig", "cython<3"]
 
 [tool.pytest.ini_options]
-asyncio_mode = "auto"
+asyncio_mode = "auto"