bugfix478 import from metplotpy.contributed instead of relative import

[ahijevyc]

Pull Request Testing

• git checkout 478-import-metplotpycontributed-instead-of-relative-import
• Install metplotpy-3.0.0rc1
• setenv METPLOTPY_BASE $SCRATCH/METplotpy
• cd $METPLOTPY_BASE
• python ./metplotpy/contributed/fv3_physics_tend/cross_section_vert.py -h

This should return a usage statement, not ImportError: attempted relative import with no known parent package.

Pull Request Checklist

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• Add any new Python packages to the METplus Components Python Requirements table.
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• Complete the PR definition above.
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[ahijevyc]

I closed an old PR linked to metplotpy version 2 and opened this one linked to main_v3.0. Along the way I over-complicated the commits, but ultimately, the PR is relatively clean; it has only 3 lines that differ from branch main_v3.0. Specifically, the physics_tend import statement is changed in three python scripts under the contributed.fv3_physics_tend directory.