MAINT: avoid pytest future warnings

[ev-br]

$ pytest --pyargs scpdt

Complains about test functions with non-None returns:

scpdt/tests/test_testmod.py::test_stopwords
  /home/br/mambaforge/envs/scipy-dev/lib/python3.10/site-packages/_pytest/python.py:198: PytestReturnNotNoneWarning: Expected None, but scpdt/tests/test_testmod.py::test_stopwords returned TestResults(failed=0, attempted=3), which will be an error in a future version of pytest.  Did you mean to use `assert` instead of `return`?
    warnings.warn(

To this end,

• restructure the tests in test_testmod to assert and not return anything
• shield testmod from pytest thinking it's a test function

[ev-br]

Minor cleanup, merging straight away.