Update CI

[CasperWA]

Summary of what has changed in this PR concerning CI:

• Use PyPI token for publishing (closes Use PyPI token in publish workflow #118).
• Add a CI test to build and install the Python package to ensure any new changes doesn't disrupt building the package.
  This is done for both the source distribution (sdist) and for the built distribution (bdist_wheel) using ubuntu-latest.
• Run CI for all PRs and only for pushes to master.
• Split up CI job build to individual jobs to have a better overview.
• Provide the flake8 linting with some teeth, in the form of not turning all errors into warnings.

Code fixes:

• Based on the flake8 linting, the files have been updated accordingly.
• Add requirements.txt to the MANIFEST.in as the "build and install Python package"-test failed due to missing the requirements.txt file.
  You might never have experienced this, if you didn't install from PyPI.

[jesper-friis]

Looks very good. Thank you!

We may consider to enforce even more flake8 linting

[CasperWA]

I'm not authorized to merge, so feel free to merge anytime :)

[CasperWA]

We may consider to enforce even more flake8 linting

Right. For this, I would consider using pre-commit to run a flake8 check on every local commit made. One might consider using the Black formatter as well?