try to get dynamic version in `pyproject.toml`

[drbenvincent]

AFTER #105...

In theory we should be able to have ONE version number in causalpy/version.py and get pyproject.toml to extract that. See the instructions here, https://packaging.python.org/en/latest/guides/single-sourcing-package-version/

But when working on #105 this was not working. So at the moment we have the version number in both causalpy/version.py and and pyproject.toml.

[OriolAbril]

I use flit in xarray-einstats and the dynamic version is working: https://github.com/arviz-devs/xarray-einstats/blob/main/pyproject.toml. (I used https://github.com/astrojuanlu/cookiecutter-pylib as template in case it helps, I don't really have any idea about the options available nor the pros/cons of each but it worked so I went with it)

[jpreszler]

I first tried to get setuptools to work, but the imports in __init__.py caused issues (either pip install -e . would fail with missing modules, or it would finish but the usage of the package would need to change, i.e. result = cp.pymc_experiment.SyntheticControl(...) wouldn't work).

Changing to flit, based on the above suggestion seems to work with minimal changes but I'm not sure show to test besides pip install -e . and python -m build and checking.

Also it seems @drbenvincent and @lucianopaz didn't like a previous attempt that changed the build system to setuptools-scm. It seems the same issues don't apply to Flit, but there could be other concerns.

[drbenvincent]

There is also bump2version as a possibility.