Add .readthedocs.yml

This is "The Modern Way" and is requoired for Python 3.12.
https://docs.python.org/3.12/library/importlib.metadata.html

As `importlib.metadata` was added in Python 3.8, and as we still support Python
3.7, we add a fallback import for older versions.

This required updating to the latest version of `pypa/cibuildwheel`.

This reached end-of-life in June 2023:
https://devguide.python.org/versions/#unsupported-versions

And attempting to build the wheel fails with an error:
```
docs: install_package_deps /project> python -I -m pip install sphinx-bootstrap-theme 'sphinx>=1.6.0'
  docs: install_package /project> python -I -m pip install --force-reinstall --no-deps /project/.tox/.tmp/package/3/lz4-0.1.dev1180+g91fe6ef-0.editable-cp37-cp37m-linux_x86_64.whl
  docs: commands[0] /project> make -C docs doctest html
  make: Entering directory `/project/docs'
  sphinx-build -b doctest -d _build/doctrees   . _build/doctest
  Running Sphinx v5.3.0

  Extension error:
  Could not import extension sphinx.builders.linkcheck (exception: urllib3 v2.0 only supports OpenSSL 1.1.1+, currently the 'ssl' module is compiled with 'OpenSSL 1.0.2k-fips  26 Jan 2017'. See: urllib3/urllib3#2168)
```

Build wheels for Python 3.12