Need to support Python 3.6

[jsquyres]

It was pointed out on the ompi-packagers list that Python 3.6 is the inbox Python in RHEL8 (and derived distros). The Python >= 3.7 check will therefore need to be downgraded to check for 3.6. There are two considerations:

1. The python scripts used in all builds (to generate some C and Fortran code) is quite simplistic and runs correctly with Python 3.6. No issues there.
2. The pympistandard module used in the docs generation process requires some functionality that was introduced in Python 3.7. However, there are backports available for Python 3.6.
   • See Support using importlib_resources (vs. importlib.resources) mpi-forum/pympistandard#27.

Since we already require additional Python modules for docs generation (Sphinx, etc.), adding a few more modules for the pympistandard module when running with Python 3.6 is not an additional burden -- we can just list these in docs/requirements.txt.

FYI @boegel