Pin style checkers and formatters

[leouieda]

Leaving them unpinned, particularly when we have so many now is going to
be a nightmare. Better to have them pinned in the environment.yml and
env/requirements-style.txt files and we upgrade them from time to time.
Fixes a false-positive from the updated RST linter.

Could be a good excuse for us to finally setup dependabot to send PRs when new versions of the linter come out.

Reminders:

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst and the base __init__.py file for the package.
• Write detailed docstrings for all functions/classes/methods. It often helps to design better code if you write the docstrings first.
• If adding new functionality, add an example to the docstring, gallery, and/or tutorials.
• Add your full name, affiliation, and ORCID (optional) to the AUTHORS.md file (if you haven't already) in case you'd like to be listed as an author on the Zenodo archive of the next release.

[santisoler]

Thanks for pinning them! I'm merging this.