Fix linter errors

[jspaaks]

List of related issues or pull requests

Refs: N/A

Describe the changes made in this pull request

Since the last time we worked on this project, more tests have been introduced which means the linter failed. This PR fixed all the prospector linter errors.

Also, isort dropped one of its arguments (--recursive), so I updated the docs and the githook.

One of the unit tests for that tests for correct removal of ReSrtucturedText comments failed, but its data file was too big to make sense for a unit test (if it fails, it's not clear what part made it fail), so I ended up removing the whole test (there are still 5 others that test more or less the same thing).

I also updated the cffconvert workflow because its target has changed its name (master branch doesnt exists anymore there), and also because the action now does a different thing than before (consistency checking between .zenodo.json and CITATION.cff v. validation of a CITATION.cff). The .zenodo.json is no longer needed, so I removed it.

I also removed Python 3.6 from the testing matrix in the workflows because that version is currently end of life.

Instructions to review the pull request

# Create a virtualenv, e.g. with
python3 -m venv venv3

# activate virtualenv
source venv3/bin/activate

# make sure to have a recent version of pip
pip install --upgrade pip wheel setuptools

# (from the project root directory)
# install howfairis as an editable package with development dependencies
pip install --no-cache-dir --editable .[dev]

# test imports
isort --check-only howfairis

# test code style
prospector

# run unit tests
pytest

[sonarcloud[bot]]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

68.8% Coverage
0.0% Duplication

[jspaaks]

Thanks for the review! :cake: