fix: updates a number of optional dependencies

[chalmerlowe]

This fix updates a number of optional dependencies.
We use a different module import process (pytest.importorskip versus unittest.skipif).

This first major commit gets the ball rolling, there are gonna be a few additional commits to cover other files.

Fixes #<issue_number_goes_here> 🦕

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[tswast]

Doesn't look like this fixture is used?

Also if it is used, could this use the constant from _version_helpers as the tests do below?