Handle python upgrades

[jaimergp]

When an environment changes python versions (e.g. from 3.9 to 3.10), the conda-pip installed packages will have to be reinstalled for the new version too. Usually, conda does that automatically as part of the solve and noarch: python handling, but since right now we are not injecting that metadata, then they will be missed.

I see two solutions:

• Enhance the post-command hook so we uninstall and reinstall the relevant PyPI packages (we are not keeping track right now!)
• Convert all PyPI wheels to conda packages (maybe via conda2whl?) so this is handled by conda proper. However, this is only feasible for noarch-y packages (py_any). Python specific builds will still need to happen, which might result in solver conflicts unless we anticipate the need for new whl2conda conversions... catch 22!

Maybe the solution is a hybrid approach, but for now I think the first option is more robust.