drop Python 3.7

[jburel]

Drop support for Python 3.7

[jburel]

hdf5 tests currently fail on windows when using Python 3.8+

[jburel]

@sbesson 9cbe240 uses the wheel produced by GS
The tests are still failing on Windows so I have not bumped to 3.8+
You approved it before my last commit (and the build was still red). Are you okay with the current state of the PR?

I will investigate the hdf issue in a follow-up PR.
Maybe an opportunity to remove our old copy of portalocker. The files stay locked leading to the failure with Python 3.8

[sbesson]

@jburel no objections to moving forward with this PR.
On the Python 3.7/Windows front, we definitely need to look into these issues separately. Removing our vendored copy of portalocker had been already mentioned in ome/omero-web#415 (comment) during the migration to upstream concurrent-log-handler in OMERO.web.

Using upstream portalocker and deprecating omero_ext/portalocker would be valuable. From a quick code review, the main locations which will need functional review are omero config, omero admin, temporary files as well as OMERO.tables.