MAINT, CI: Update Ubuntu 18.04 to Ubuntu 20.04

[charris]

Github actions will drop support for 18.04 April 1, 2023. There are now sporadic test cancelations due to warning brownouts, and the NumPy 1.24.x release will overlap that date, so increase the Ubuntu version.

The main change here is that there is no support for gcc-6 in Ubuntu 20.04, so testing with that version is dropped.

[charris]

Hmm, build error running "full" on azure:

numpy/core/src/multiarray/einsum_sumprod.c.src:1264:1: internal compiler error: in coverage_begin_function, at coverage.c:656

[charris]

Coverage fails in Ubuntu 20.04, but works in 22.04. Unfortunately, there are a lot of error like this:

numpy/core/src/multiarray/nditer_constr.c:2004:32: error: writing 8 bytes into a region of size 0 [-Werror=stringop-overflow=]

See https://github.com/numpy/numpy/actions/runs/3474938876/jobs/5808717885 for more. The -Werror=stringop-overflow option seems to be new with that version (new in gcc-10), IIRC, we used to get warnings., In any case, we should probably fix those warnings. For now, I'm just going to disable the coverage option.

[seberg]

LGTM, but not merging since I am not sure if you want to take a shot at fix the coverage warnings this first.

Do we need to document that GCC 6 is not officially tested anymore?

[charris]

fix the coverage warnings this first

Since that is fixed in Ubuntu 22.04, I'm not going to worry about it. The warnings in that version are unrelated and due to the stringop-overflow warning, so once that is taken care of we can re-enable the coverage. I don't think the lack of coverage testing is a blocker.

Do we need to document that GCC 6 is not officially tested anymore?

Good point. I'll add a release note, but not sure what section to put it in.

[seberg]

Maybe just "compatibility" if we are talking release notes? Seems OK, and we can still move it if desired.

[seberg]

OK, lets put this in, thanks Chuck. Was there something to followup on if the warning fixes work out?