Add custom build backend to support build args (#328)

This implements a custom build backend, inspired by the
[solution used by Pillow.](https://github.com/python-pillow/Pillow/pull/7171)

install-option is deprecated and was removed with pip 23.1. The
comonly accepted solution seems to be to define a custom build
backend for now
https://github.com/pypa/setuptools/issues/2491#issuecomment-1589764230

This commit changes the usage from `--install-option` to `--config-settings`.

tox.ini

@@ -7,10 +7,11 @@ deps=
     pkgconfig
     Jinja2
     pytest
+    pytest-asyncio
     cython<3
 
 commands =
-    python setup.py install
+    pip install .
     py.test {posargs}
 
 setenv =