Convert namespace tests to pytest

[aucampia]

Summary of changes

This change refactors the unittest based tests for namespaces to
pytest based tests. This is primarily in preparation for adding a
new tests.

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Checked that all tests and type checking passes.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[ghost]

My bad, I hadn't spotted this PR and I should have. In merged PR #1788 test/test_namespace.py has been moved to test/test_namespace/test_namespace.py

Have I been a bit heavy-handed in my approach to the migration of unittests to pytest? I've been refactoring out all of the unittest stuff. Is retaining the unittest class structure a more elegant approach?

[nicholascar]

Have I been a bit heavy-handed in my approach to the migration of unittests to pytest? I've been refactoring out all of the unittest stuff. Is retaining the unittest class structure a more elegant approach?

Nah, kill all that awful Unittest non-Pythonic code! I admire your diligence in making the change! If I had done the test restructuring, I likely would not have done the refactoring too, but it's better off done.

[aucampia]

My bad, I hadn't spotted this PR and I should have. In merged PR #1788 test/test_namespace.py has been moved to test/test_namespace/test_namespace.py

@gjhiggins it is fine 😄 I tagged you for review, not because you were moving it, quick enough to rebase.