Update test matrix

### 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](https://github.com/jupyterhub/batchspawner/pull/247#issuecomment-1227763489):

> 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](https://github.com/jupyterhub/jupyterhub/blob/main/.github/workflows/test.yml#L76) 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 ?