Unpin pyyaml version

[tcompa]

In #812 we are pinning pyyaml to 5.3.1 (downgrading from 5.4.1), to avoid problems due to the recent cython=3 release:

• "AttributeError: cython_sources" with Cython 3.0.0a10 yaml/pyyaml#601
  (notice the endless list of issues referencing to this one)
• Error installing Pyyaml==5.4, Cython_sources yaml/pyyaml#724
• [WORKAROUND] Unable to build PyYAML < 6.0.1 from source or sdist yaml/pyyaml#736
• Request for a v5.4.1.post1 release to set Cython<3  yaml/pyyaml#728

Luckily we only use this dependency in the dev/docs groups, so that it doesn't really matter for our package. Still let's unpin it (in the future) as soon as a solution is found.

For the record:

• https://github.com/fractal-analytics-platform/fractal-server/security/dependabot/14