Fix for gha:validation error in Github actions

[lennertvandevelde]

Summary of changes

Test to check if this fixes the issue described here in the DEV mailinglist. Someone who knows in depth what these validation tasks do will have to check if they still work as intended, this merely fixes them not running.
Approval needed to test the workflows: https://github.com/RDFLib/rdflib/actions/runs/9466926485

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.

[nicholascar]

@lennertvandevelde I merged master into this PR to bring this up-to-date but was that the wrong thing? Have I now broken your poetry.lock update?

[ashleysommer]

@nicholascar
I just checked the diffs, your commit didn't affect the poetry.lock file.

[lennertvandevelde]

Last error is this one:

  py38-extensive-docs: commands[3]> poetry run python -m mypy --show-error-context --show-error-codes --junit-xml=test_reports/3.8-ubuntu-latest-mypy-junit.xml
  rdflib/plugins/sparql/algebra.py: note: In member "convert_node_arg" of class "_AlgebraTranslator":
  rdflib/plugins/sparql/algebra.py:1010: error: Subclass of "str" and "Expr" cannot exist: would have incompatible method signatures  [unreachable]
  rdflib/plugins/sparql/algebra.py:1011: error: Statement is unreachable  [unreachable]
  Found 2 errors in 1 file (checked 394 source files)
  py38-extensive-docs: exit 1 (11.95 seconds) /home/runner/work/rdflib/rdflib> poetry run python -m mypy --show-error-context --show-error-codes --junit-xml=test_reports/3.8-ubuntu-latest-mypy-junit.xml pid=2322