Update setup.py to require python >= 3.7

[ahnitz]

Arxviz due to its requirement on xarray>=0.16.1 (and possibly for internal reasons as well) in practice has a hard requirement on the python version of >=3.6. However, it is not declared at the moment in the setup.py so the package resolution doesn't notice this through pypi until attempting to satisfy the xarray requirement.

[OriolAbril]

Thanks, we should first merge #1430 though, otherwise CI will attempt to test on python 3.6. Thanks for the PR, we were planning on adding this to the next release, I am adding this PR to the milestone

[ahnitz]

@OriolAbril I believe python3.6 should in fact be OK (unless other PRs have already implicitly removed 36 support)

[OriolAbril]

Oh, I missed the equal, we should drop support for python 3.6 following nep 29, most of the packages in the pydata ecosystem have already done so: https://github.com/numpy/numpy/blob/main/setup.py#L407, https://github.com/scipy/scipy/blob/master/setup.py#L525, https://github.com/pandas-dev/pandas/blob/master/setup.cfg#L37, https://github.com/pydata/xarray/blob/master/setup.cfg#L75, https://github.com/pymc-devs/pymc3/blob/master/setup.py#L85...

Can you update the PR?

[ahnitz]

@OriolAbril Yes, done.