Add pytest to CI and align Python versions

[rogeralsing]

Update the GitHub Actions workflow to run pytest and linting with the supported Python versions (>=3.10).\n\nAcceptance criteria:\n- Workflow matrix uses Python 3.10-3.12 to match pyproject.toml.\n- pytest runs in CI (install deps as needed).\n- pylint still runs.

[rogeralsing]

CI is currently failing on master due to the Pylint workflow running default checks over the whole repo (exit code 30).

Example failing run: https://github.com/asynkron/Asynkron.TextLayout/actions/runs/21017719475

While tightening Pylint to error/fatal only (E/F), it also surfaced a real error in src/textlayout/parsing/pdf_text_extractor.py: missing import for 're' (E0602). Fixed as part of PR #6 (also aligns the matrix to Python 3.10-3.12).

[rogeralsing]

Follow-up: PR #6 fixes the current failing CI on master (Pylint workflow + missing import). It does not add pytest yet.