Improve doctests handling

[HelgeKrueger]

Note: I don't expect this to be easily doable in this project. It would either belong into pytest or maturin. This is just where this issue arose for me.

This issue arose while implementing #72

By adding

[tool.pytest.ini_options]
addopts = '--doctest-glob="*.rst"'

to pyproject.toml, one can run the docstrings in docs/index.rst using pytest. See here.

Unfortunately, this doesn't run the doctests in src/lib.rs.

Question: Can one adopt the tooling to also run these?

[messense]

Maybe we can run doctests using https://github.com/twmr/pytest-sphinx?