TST: fixup CI, drop EOL Python versions

[neutrinoceros]

address 3 problems I encountered with CI:

• parametrization on Python version didn't work: all jobs were effectively running Python 3.12.3
• selected ubuntu images are not available any more
• despite support being claimed for Python 2.7 and 3.5-3.8 (which are all end-of-life), Python 3.9 was effectively required at least since 18cda8f (which used the | operator to join dictionaries, only supported since 3.9)

Since actually testing on older Pythons is actually becoming very hard these days, I propose simply dropping them.

[neutrinoceros]

I would also suggest adding the pull_request trigger on CI, so this could be properly validated

[marshallward]

Thanks for reporting this. I really bungled the testing here, and should have known better than to use d1 | d2 in supposedly Py2 compatible code. (I even had a |= operator in there!).

The intention was to do one last release with Python 2 compatibility, then do a major release and drop it entirely. I guess that plan is not exactly an option anymore.

I'd like to think on this a little before moving ahead. But as it stands, your PR is entirely correct to drop support (and, of course, fix the testing).

[neutrinoceros]

No rush on my end !

[marshallward]

Effectively resolved by #177 although we will come back to the eventual removal of Py2.7 support.