Unify/stagger CI requirements.txt

[nicolaskruchten]

We could:

• drop tox
• have different requirements.txt files per Python version
• merge "optional" in there
• unify the docs-build requirements.txt with that
• pull PX tests out of core and move those tests to optional
• run the docs as part of optional
• rename plot_ly to chart-studio

[nicolaskruchten]

Good enough! Not sure if the docs unification is super-worth it ATM :)