MRG, ENH: Add array-spacing plugin and reorganize deps

[larsoner]

Playing around with CIs a bit:

1. Add flake8-array-spacing plugin. It allows us to remove some # noqa lines, which is nice. Not 100% sure we want this, but I want to see if it works at least.
2. Remove pytest, flake8, and related things from the mne env and requirements.txt. Now these files only contain what is necessary to get 100% functionality out of MNE-Python.
3. Create requirements_testing.txt and doc/requirements.txt with the additional requirements for running tests and building docs, respectively.

This simplifies our CI code a bit, which is nice. I'm not expecting the CIs to come back happy on the first try, but folks should feel free to have a look and see if they're happy with the overall structure for when I can get the CIs to cooperate.

[drammock]

changes look reasonable. only thing I'm pondering is whether the filenaming is as clear as it could be, and the slight weirdness of having one of the requirements files under doc/

[larsoner]

I went back and forth on the naming, too. But I figured having one under doc/ was better than another root-level file. But we don't have a better place for the _testing.txt one. I guess I could rename tools to testing...?

[drammock]

I figured having one under doc/ was better than another root-level file.

A bloated root-level doesn't bother me, FWIW, as long as the filenames are well organized. requirements.txt, requirements_testing.txt, and requirements_docs.txt seems fine to me. I'd also change server_environment.yml to environment_server.yml or perhaps environment_headless.yml

[larsoner]

I'd also change server_environment.yml to environment_server.yml or perhaps environment_headless.yml

Let's hold off on this one, I'm hoping that maybe we won't need it after pyvista/pyvista#824 (?)

[agramfort]

thx @larsoner

we'll deal with notebook test failure next week