1. the pip requirements_* files seem to be multiplying... is there need/value in specifying them in separate text files rather than directly in setup.py?

2. for the install shorthands, to me it seems the fewer options the better; something like:

• pip install mne # base
• pip install mne[hdf5] # + hdf5 / export deps; other shortands could be "io" or "export"
• pip install mne[dev] # everything above + doc and testing deps; could also be "all"

so if we find ourselves wanting to add more and more install shorthands as our dependencies evolve, I'd rather think about how to shoe-horn new deps into one of those 3 levels.

Originally posted by @drammock in #10199 (review)