Add @StevenMaude's GitHub Workflow Action test on Windows and Ubuntu (the proper way)

[ParticularMiner]

(as per @StevenMaude's instructions)

[ParticularMiner]

During the merge, the GitHub Workflow action tests were triggered as expected. See test details by clicking the View details button above associated with this merge commit. In summary,

• All Ubuntu-run tests succeeded on master (my synced copy of Bergvca/string_grouper:master).

• All Windows-run tests failed on master.

• Failures were due to a curious platform-dependent numpy int datatype mismatch (that occurs only on Windows as far as we know).