remove conda-based install

[leifdenby]

This PR fixes the ci/cd by:

• adding numba dependency to setup.cfg (rather than environment.yml). It also deletes environment.yml and the conda-based ci/cd setup since we don't need conda-based dependencies, since everything is pip-installable)
• pinning python version in the linting step to python3.8 (there is some major change for importlib that changed with python3.12 that was being used by default in the ci/cd pipeline)

[leifdenby]

Great, thanks so much @leifdenby for fixing this. The only note is that we might eventually consider updating our ci/cd pipeline so it supports the newer version of importlib, so we're not stuck with legacy python versions.

Yes, true. Although I think there's something upstream going wrong with pre-commit and python3.12 for now.

Another thing on importlib is that we might want to consider dropping python3.7 support since its version of importlib doesn't have the importlib.metadata part which can be used for retrieving package version stuff. That would allows us to clean up the setup.cfg, setup.py and pyproject.toml mess too... https://docs.python.org/3.9/library/importlib.metadata.html