PR summary

Test some currently untested GUI methods. (get_window_title, smoke test for remove_rubberband.)

Replace unittest with pytest.

Add get_window_title method to webagg backend.

Remove tk-backend-marks (allowing tests to be run). Importability is checked in _isolated_tk_test.

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• Plotting related features are demonstrated in an example
• New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines