Fixes to docstring and CI jobs.

[mairanteodoro]

This PR fixes the CI jobs by making the WEBBPSF_PATH environment variable available while running the jobs.

This PR also fixes an issue with the returned type specified in a docstring.

Checklist

• [ ] added entry in CHANGES.rst (either in Bug Fixes or Changes to API)
• [ ] updated relevant tests
• [ ] updated relevant documentation
• [ ] updated relevant milestone(s)
• [ ] added relevant label(s)

[codecov[bot]]

Codecov Report

All modified lines are covered by tests :white_check_mark:

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