Bugfix webbpsf marker

[WilliamJamieson]

spacestelescope/stcal#225 is still having failures due to webbpsf. This is due to the fact that a conftest.py was used to register the webbpsf marks and not registering them as a pytest plugin. As a result, these markers are not defined when running pytest from a different location as the top-level conftest.py is not available.

This PR resolves this issue by wrapping the webbpsf marker code into a pytest plugin. Note that webbpsf should probably incorporate this code as it is solving an issue cause by using webbpsf; however, these changes should go in for now to assist upstream packages with downstream testing against romancal.

Checklist

• added entry in CHANGES.rst under the corresponding subsection
• updated relevant tests
• updated relevant documentation
• updated relevant milestone(s)
• added relevant label(s)
• ran regression tests, post a link to the Jenkins job below. How to run regression tests on a PR

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

see 1 file with indirect coverage changes

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