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)

Hello @datapythonista! Thanks for updating the PR.





















Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on December 29, 2018 at 12:35 Hours UTC Is this still active? yes, I need to investigate why tests are taking longer in this branch, which is weird, but would be great to have this merged after that, it cleans the CI configuration significantly closing as stale.