Update test matrix

[ryanlovett]

Proposed change

Update text matrix to be more inline with jupyterhub and in-use distributions.

#247 was failing in python 3.5 but not 3.9. I saw that 3.5 had not been supported in JupyterHub for a while so asked whether 3.5 could be dropped. @mbmilligan said:

So yes, I think I'm okay with dropping testing support for Python 3.5. I wouldn't drop 3.6 support for a while yet though. As that's currently the newest Python that can easily be installed on those CentOS 7 systems, I think we're stuck with it until those systems go EOL in 2024.

This suggests changing the oldest version batchspawner supports from 3.5 to 3.6.

Alternative options

Do nothing, and ensure future PRs are compatible with python 3.5 too.

Who would use this feature?

Contributors who don't want to support older pythons.

(Optional): Suggest a solution

Should the matrix be expanded to support everything JupyterHub does, plus 3.6? That is, batchspawner's tests are on 3.5 and 3.9 right now, so should the set be 3.6 and 3.9, or 3.6 .. 3.11 ?

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[willfurnass]

NB folks still on EL7 can get Python 3.8 from the rh-python3.8 RedHat Software Collection (more info).