Fix invalid python imports in pytests

[avivajpeyi]

Close #1454

1. Fixed pytest import errors by removing from py_tests... style imports.
2. Replaced silent cosmic-integration import fallbacks with a clear setup error message (install with -e . / -e '.[dev]').
3. Made mass-range handling work for tiny test datasets and aligned DCO naming (BHBH).

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[LiekeVanSon]

Looks good to me! Note sure, but I might have been to blame introducing this in the latest update to the CosmicIntegration scripts, if so, sorry, but thanks for cleaning this up!

Related: do we maybe want a changelog for the python scripts as well? (this might help prevent such incidents in the future)