These CI jobs are hard to navigate with several levels of indirection (a workflow file, then an action file, then two tools/travis-*.sh scripts), and now that we have to move a lot of jobs over to Meson it's a good time to clean up. The build_test.yml is particularly messy, and contains 22 jobs which is too much (quite a few are unnecessary too).

A few thoughts:

• The windows_meson.yml and windows_clangcl.yml can be merged, and named windows.yml
• It's useful to keep unusual / niche platform jobs in separate files. So let's keep emscripten.yml, cygwin.yml, linux_musl.yml and linux_compiler_sanitizers.yml.
• Let's break up build_test.yml:
  • linux_simd.yml: all the SIMD related jobs (~8 jobs) and the old_gcc one
  • linux_blas.yml: all the BLAS/LAPACK related variations we're testing (should end up with ~6 jobs)
  • linux.yml: testing across Python versions (PyPy, python-dbg, a pre-release Python in summer time), build-via-sdist, run benchmarks, measure code coverage, and other build options like relaxed-strides. This will also fold in linux_meson.yml.
• The SIMD tests are the only ones that may use a .sh script, for the checks that are now in travis-test.sh. For the rest, those shell script are just hard to read cruft.
• I think we'd like a summary of what the larger files to at the top. In particular for SIMD, BLAS and the main linux.yml ones, that would be really helpful.
• No more matrix'ed tests on a single variable, that's too much repetition of the exact same thing. We must be testing Python 3.9 + OpenBLAS + ubuntu-latest with its default GCC version more than 5 times by now, and it doesn't do much except heat the planet.