dev: pin pytest<8.0.0

[machow]

This is a temporary patch to fix CI runs. It pins pytest to be <8.0.0.

Seems like the long term fix is likely here:

https://docs.pytest.org/en/7.1.x/explanation/goodpractices.html#tests-as-part-of-application-code

Here is the error encountered on test collection:

_________________ ERROR collecting quartodoc/tests/test_ast.py _________________
import file mismatch:
imported module 'quartodoc.tests.test_ast' has this __file__ attribute:
  /opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/quartodoc/tests/test_ast.py
which is not the same as the test file we want to collect:
  /home/runner/work/quartodoc/quartodoc/quartodoc/tests/test_ast.py
HINT: remove __pycache__ / .pyc files and/or use a unique basename for your test file modules