Decrease the maximum allowed Python version for gmxapi.
Update setup.py to require Python version <3.9. We should allow ourselves
a chance to review and respond to changes with the Python 3.9 release
before asserting support.
Changes with Python feature releases aren't likely to affect scientific
results, but could cost valuable computing resources. Historically,
gmxapi updates for feature releases have included Python base class
adjustments (responding to changes in the `typing` module with Python
3.7) and pybind11 header updates (responding to Python C API changes).
Resolves #3175
Change-Id: I7805964669ac74111eecf24402e651343f7908c0