For Python 3.11, use tomllib for tests

[musicinmybrain]

Avoid a test dependency on the PyPI toml package.

Fixes #2027.

Checklist

• I have read the contributing doc.
• I have included a link to the relevant issue number. N/A
• I have checked to ensure there aren't other open pull requests
  for the same issue.
• I have written new tests for these changes, as needed. Covered by existing tests.

[musicinmybrain]

The toml case is written as

    pyproject_contents = toml.load(pyproject_path)

in this PR, which is concise and similar to what we had before. We could have instead written

    with open(pyproject_path) as pyproject:
        pyproject_contents = toml.load(pyproject)

which would more directly parallel the tomllib case. However, it’s slightly more verbose, and it wouldn’t allow us to share the part that opens the file, since tomllib wants the file opened in binary mode and toml doesn’t, so there’s no real advantage in writing it that way.