CI: remove setup.py based jobs from GitHub Actions and run less on merge to main

[rgommers]

This also improves the python-dbg build, as well as makes visible that that job tests ATLAS as the BLAS/LAPACK library (that worked pleasantly well).

There was one failing test, test_signaltools.py::test_lfilter_bad_object. That's super niche and we've had problems with it before, so I just skipped it when running in a debug interpreter.

Running CI on merges of PRs to main is a waste of resources, and I find myself writing [skip ci] in merge commits a lot. I disabled those to save resources. We did that for NumPy too recently, and that has worked fine.

[tupui]

Thanks Ralf. +1 to not run checks on merge. I just have 2 questions.

[tupui]

The new job is green. But not sure about the symbol hiding part. Can you confirm?

  ./tools/check_pyext_symbol_hiding.sh build
  shell: /usr/bin/bash -e {0}
  env:
    CCACHE_DIR: /home/runner/work/scipy/scipy/.ccache
    INSTALLDIR: build-install
find: ‘build’: No such file or directory

[rgommers]

Well that's not helpful, on Windows:

 E   AttributeError: module 'sys' has no attribute 'abiflags'

Guess I should have read the docs, which say it's POSIX-only. Not sure if there's a better way to determine whether we're running a debug build of the interpreter - will look at that.

[rgommers]

The new job is green. But not sure about the symbol hiding part. Can you confirm?

Good catch, thanks. I'll need to fix that up.

[tupui]

All green and tests seems ok now. Lets get this in. Thanks Ralf.

[rgommers]

Very satisfying I must say, this PR 😄

[tupui]

Totally, almost nothing is running now 😃