Hepunits

- dependabot.yml file added.
- GitHub Actions pre-commit version updates.

**First Python 3 only release.**

- Improved typing support.
- Added noxfile for easy running.
- Pre-commit updates.

- Python 3.10 added to the CI.
- Pre-commit version updates.

- Better documentation via `help(...)`.
- CI improvements and updates based on Scikit-HEP org guidelines.
- Re-added coverage tests, now via GHAs.

- More and better documentation in the README.
- CI improvements based on Scikit-HEP org guidelines.

- Boltzmann constant added (exact constant, i.e. with no uncertainty, taken from the PDG 2020 review).
- Several (sub-)units introduced.
- Package now available in Conda-forge.
- CI updates.