Drop support for Python 2

[benmwebb]

python-ihm still works on a Python 2.7 system, but this requires some amount of maintenance, and we can't use some nicer Python 3 features like type annotations or f-strings. It is also getting harder to actually test the Python 2 codepaths in CI or on current workstations.

Plan to drop Python 2 support and clean up the codebase. Are there any uses of python-ihm (or python-modelcif) that still need Python 2? IMP uses it but as of the last stable release is now Python 3 only. @brindakv, any RCSB Python 2 workflows that use python-ihm? (I would assume not)