Include all test files in sdist

[millerdev]

Fixes error due to no tests found while building and publishing to PyPI. This change was intended to be part of #236. Prior to this, the test files in the source distribution were incomplete.

Sample failure:

       + sh -c 'cd /project; python -m unittest'
  ----------------------------------------------------------------------
  Ran 0 tests in 0.000s
  NO TESTS RAN
                                                              ✕ 3.16s
Error: Command ['sh', '-c', 'cd /project; python -m unittest'] failed with code 5. 
Error: Process completed with exit code 1.

I missed this MANIFEST.in change when I was renaming tests.yml back to pypi.yml after I had completed testing the action. See testing process in the description of #236 for more details on how that was done.

[millerdev]

It is possible to use Workflow Dispatch to run the Build wheels and publish to PyPI action on this branch now that pypi.yml is on the master branch.

✅ It works now - evidence on PyPI