pin astroid to <4 to avoid docs build failures

[d-v-b]

This PR adds an empty file to probe recent docs build failures. It should not be merged.

This PR adds astroid<4 to the docs dependencies, which allows the docs to build properly.

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/user-guide/*.rst
• Changes documented as a new file in changes/
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[d-v-b]

closes #2797

[d-v-b]

pre-commit failure can be ignored. the spellchecker wants "astroid" to be changed to "asteroid".

[dstansby]

Probably don't want pre-commit to be failing on all other PRs, so I added some config to ignore astroid. Otherwise 👍 , so I'll merge if CI passes to unblock CI on other PRs.