centralize specification of python versions

[mschrimpf]

Since we allow users to provide their own environment.yml (#31), they can also specify a python version and we require that version to be aligned with the library's supported versions.

We also specify the python version in the pyproject.toml, .readthedocs.yml, and the .travis.yml.
(Also some tests but I think those are fine.)

Does it make sense to unify the python version specification? Or is this acceptable?