Add pytest

[PythonFZ]

Currently, the builds are published all the time.
This introduces running a quick test, before publishing.

• should the test also be included into the .github/workflows/actions.yml ?
• bump version to 21.1.4

The motivation for this PR is, that v21.1.3 on pypi has some broken wheels whilst v21.1.2 works, which makes no sense, since there were no real changes between them that could have an effect on that v21.1.2...v21.1.3 Thus I assume there was some unexpected behavior in the CI which I can't explain but we can test.

[lmiq]

I'll merge this.

I'm reluctant to add this test to actions.yml because then this distribution becomes a blocking feature for the releases. That's the kind of thing I can't handle alone.

Given this, I wander if this build really has to live within the packmol repository, or if it could be in a different repository with a simple update whenever a new packmol version is released. What do you think? That might be easier to maintain in the long run.