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`.

pyproject.toml

@@ -1,5 +1,7 @@
 [build-system]
 requires = ["setuptools", "wheel", "pkgconfig", "cython<3"]
+build-backend = "backend"
+backend-path = ["_custom_build"]
 
 [tool.pytest.ini_options]
 asyncio_mode = "auto"