Drop Python 3.7 support?

[ml-evs]

Several packages we depend on are dropping Python 3.7 support, driven I guess by NumPy's deprecation timeline: https://numpy.org/neps/nep-0029-deprecation_policy.html

Dependabot does not show us updates that use incompatible Python versions, so we might have to just follow the crowd here and drop 3.7 support sooner rather than later ourselves too.

[ml-evs]

I'm not actually sure how dependabot interacts with different Python versions. It might be grabbing Python 3.7 from our unloved Pipfile:

optimade-python-tools/Pipfile

Line 12 in 79a9d52

python_version = "3.7"

I would suggest we drop the Pipfile if no-one we know is using it.

[JPBergsma]

I think It would be best to ask the users whether they would have a problem if we dropped support for python 3.7.
Perhaps we could do this at the upcoming Optimade meeting.
It would also be nice if we could somehow give a deprecation warning that a future version of optimade will no longer support python 3.8. This should give users some time to switch to a newer version of python.

I am currently still using python 3.7 in my optimade environment. But I do not think it should be a problem to upgrade to 3.8

[ml-evs]

Good plan, I think we should add a deprecation warning now before we release 0.17, as we will probably want to drop support entirely by the time 0.18 comes around.

[JPBergsma]

That sounds good, issue the deprecation warning in version 0.17 and then drop support for python 3.7 in version 0.18.