ci: fix tests and readthedocs

[wpbonelli]

Add pytest as a ReadTheDocs dependency. Needed because the docs build procedure updates timestamps with distribution/update_version.py, which uses pytest. ReadTheDocs-hosted builds have been failing since #1069 because of this. The GitHub Actions docs.yml workflow has passed in the meantime because it installs the Conda environment.yml before installing extra docs deps.

Also address a few issues with external model test selection introduced in or noticed after #1130

• Explicitly deselect large/example model tests in commit-triggered CI, since the convention is to only run these nightly.
  • With the recent addition of a default search location for devtools model-finding fixtures, test_z03_*.py runs large/example model tests if the model repos are found next to the modflow6 repo. Starting with the recent release automation work, the examples repo was installed by commit-triggered CI and used by dist script tests, but commit-triggered CI now only needs modflow6-testmodels, as was the case originally, so this PR goes back to not installing the examples. As such deselecting is not strictly necessary, but it seems best to be explicit anyway.
  • Installing the examples added several minutes of runtime so this will have the benefit of speeding up CI runs
• Before refactor(tests): use devtools fixtures #1130, test_z0*.py scripts implicitly excluded models in nested directories due to the implementation of the get_example_dirs() function. Devtools finds namefiles recursively, so this PR explicitly excludes nested models in test_z03_examples.py to keep the set of tested models unchanged.
• Fix should_compare() function in autotest/conftest.py (bug caused the large model test CI failures starting after 1130)
• Add markers to pytest.ini

Note: if tests are being improperly skipped in a local dev environment after pulling recent changes, modflow-devtools likely needs to be updated. If external model repositories (e.g. modflow6-testmodels, modflow6-examples) live side by side with the modflow6 repo, they will automatically be found by devtools >= 0.1.3. If the repos are somewhere else, the REPOS_PATH env var can be set to point to the parent directory containing them. If the repos can't be found, external model tests (z0* etc) will be skipped. This is documented in DEVELOPER.md with more info in the devtools docs