Check for paths in `__contains__`

[TomNicholas]

• [x] Closes #240
• [x] Tests added
• [x] Passes pre-commit run --all-files
• [ ] New functions/methods are listed in api.rst
• [ ] Changes are summarized in docs/source/whats-new.rst

[TomNicholas]

This currently only works for absolute paths - perhaps it should also check for relative paths too...