specify data-related extras for pip install

[drammock]

This does the easy part of what #9821 was trying to do:

• allows pip install mne[data] to automatically install dataset-fetching dependencies tqdm and pooch in addition to numpy and scipy

[agramfort]

you'll need to update the doc and maybe some CI too no?

thx @drammock

[larsoner]

you'll need to update the doc and maybe some CI too no?

Easiest might be to modify these lines:

mne-python/azure-pipelines.yml

Lines 241 to 245 in 22728ce

	- script: pip install -e .
	displayName: 'Install MNE-Python dev'
	- script: pip install --progress-bar off -r requirements_testing.txt -r requirements_testing_extra.txt codecov
	condition: eq(variables['TEST_MODE'], 'conda')
	displayName: Install testing requirements

To use a single pip install -e .[testing] codecov line or so

[larsoner]

Nice cleanups in there, too!

LGTM +1 for merge assuming CIs come back happy

[larsoner]

Thanks @drammock