WIP/CI: Clean up to test calls
**DO NOT MERGE**: As single tests do not fail in a deterministic way, I'll rerun the CI several times to see that the `--dist` option is fixing the concurrency problems as expected.

- Using `--dist=loadscope` instead of calling pytest twice to not run `single` tests in parallel
- Renaming `single` marker to `serial` to keep consistency with pytest naming
- Moving `pytest` common arguments to `setup.cfg` so they don't need to be repeated in every call in the CI or in the local environments
- Removing shorcut scripts to call `pytest` with the parameters added to `setup.cfg`
- Made the `run_tests` script multiplatform by reimplementing it in Python, so all the calls are the same (which avoids duplicate code, and could allow changing the test locale in Windows)
