Update requirements.txt to include cellpose

[hinerm]

[hinerm]

@bethac07 @alicelucas this was necessary because without cellpose installed, other CellProfiler-plugins break (i.e. RunImageJScript) because the runcellpose.py setup fails.

[bethac07]

Hmmm, each plugin should be independent in its loading, and I can't confirm either on my system - I had both modules importable, pip uninstalled cellpose, and it stopped loading in but RunImageJ did not...

[hinerm]

@bethac07 thanks for looking at it. I was getting runtime errors from pyimagej that seemed to go away after installing cellpose, but I can't reproduce either now so maybe it was a RunImageJScript configuration issue. Sorry for the distraction!

[ErinWeisbart]

Closing this PR because we are currently overhauling how we handle dependencies (FYI @callum-jpg, in case this helps you)