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?

Pipfile

@@ -4,7 +4,7 @@ url = "https://pypi.org/simple"
 verify_ssl = true
 
 [packages]
-Cython = "*"
+Cython = "<3"
 Jinja2 = "*"
 black = "*"
 flake8 = "*"

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"

requirements.txt

@@ -1,6 +1,6 @@
 jinja2
 black
-cython
+cython<3
 flake8
 setuptools
 pkgconfig

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py37,py38,py39,py310
+envlist = py37,py38,py39,py310,py311
 skipsdist = True
 
 [testenv]
@@ -7,7 +7,7 @@ deps=
     pkgconfig
     Jinja2
     pytest
-    cython
+    cython<3
 
 commands =
     python setup.py install