fix: readthedocs failure with poetry 1.8

[edmondchuc]

Summary of changes

There's some changed behaviour in readthedocs CI environment and poetry v1.8.0 and above. Running a poetry install does not install into the correct python virtual environment that is set up by readthedocs.

This PR fixes the above issue by performing a poetry export into a requirements.txt file and then installing the dependencies and rdflib itself via pip, ensuring everything is installed into readthedocs virtual environment.

This should unblock the Dependabot PR #2743 that updates poetry to the latest version (1.8.2).

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Checked that all tests and type checking passes.
• If the change adds new features or changes the RDFLib public API:
  • Created an issue to discuss the change and get in-principle agreement.
  • Considered adding an example in ./examples.
• If the change has a potential impact on users of this project:
  • Added or updated tests that fail without the change.
  • Updated relevant documentation to avoid inaccuracies.
  • Considered adding additional documentation.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[coveralls]

coverage: 90.968% (+0.009%) from 90.959%
when pulling b51b42a on fix/poetry
into 9834665 on main.