Fix `main` branch used by diff-cover to work in more situations

[0cjs]

WARNING: to make manual checks easier, there is a temporary "test" commit on this dev branch that must be removed before merging. (@0cjs will do this on request, or you can do it yourself if you don't want to wait.)

See the commit message for a detailed description of the change and its rationale.

To see that the change is working, you can run tox -e py11 -- tests/config/test_of_types.py or similar; you should see that diff-cover finds that the to-be-removed test commit has 100% coverage of its new code. There were previously no automated tests to show that diff-cover is working correctly, and I have not added any because that seems very difficult to do and not worth the cost.

Checklist:

• linter: not needed; this code is not covered by it
• descriptive PR text: not copied from the commit message because DRY
• tests validating fix: see above
• news fragment: per commit message, not needed or wanted because it's not a user-visible change (or change to the tox program at all)
• updated/extended documentation: there's a suggestion in the commit message, but as it requires further discussion, I am leaving that for a separate PR.

[0cjs]

Checks notes:

• The timeline protection needs to be removed unless you really want a changelog entry.
• The pre-commit.ci checks will pass when the test commit is removed; they're formatted the way they are to make it easier to see what they're doing while they're there.
• Tried to fix the type check problem, but I'm running into issues with a robot heavily modifying my code. I'll look into this.

[gaborbernat]

Closing as seems it stalled, we can reopen if you get time again for it.