• fixes containment should only fail for StrictUndefined, not regular Undefined #1448

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
  • There was no docs added when this was set, so there shouldn't be doc to remove or change.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
  • No docstring changed
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.