Hepunits

Support for Python 3.9 added.

- Physical constants taken from the PDG 2020 review for what concerns exact constants (i.e. no uncertainty).
- Several sub-units introduced.

Minor updates for `pre-commit` and to `setup.cfg`, nicer errors and formatting.

This release changes much of the testing and organization internally, but has minimal user impact. The one new feature is support for static type hints; you can now use MyPy or another static type hint checker with hepunits. This does not (yet) add special type checking to the units, but just makes sure that units are transparently seen as floats so downstream packages can implement type checking.

More units and aliases for time-related units.

Documentation updates.