🔧 --no-showlocals to negate addopts w/ --showlocals

[tony]

Fixes #10381

This is necessary for addopts=--showlocals where individual test runs need to not show locals.

• Include documentation when adding new features.
• Include new tests or update existing tests when applicable.
• Allow maintainers to push and squash when merging my commits. Please uncheck this if you prefer to squash the commits yourself.

If this change fixes an issue, please:

• Add text like closes #XYZW to the PR description and/or commits (where XYZW is the issue number). See the github docs for more information.

• Create a new changelog file in the changelog folder, with a name like <ISSUE NUMBER>.<TYPE>.rst. See changelog/README.rst for details.

[Zac-HD]

Thanks for adding this!