Remove upstream champ usage and re-enable Python 3.7/3.10 testing.
Roughly speaking, this makes all the `champ_usage` tests ~10% faster on average and eliminates multiple warnings from the upstream champ code (numerical optimization issues, deprecation warnings, etc.).
It also makes our CHAMP code more robust and eliminates some non-deterministic failures that were observed in the upstream code. These used to cause failures in at least one CHAMP test case occasionally.
We keep the champ dependency where possible (i.e. for Python <3.10) just in case the user wants to run some of the related experiments.
**Full Changelog**: https://github.com/ragibson/ModularityPruning/compare/1.1.3...1.2.0