MRG, MAINT: Bump requirements, remove cruft

[larsoner]

1. Bump requirements to those ~2 years old (this includes Python 3.7 as minimum, SciPy et al. have done similarly so I think it's okay)
2. Remove things from fixes.py that required old versions (np.einsum, np.take_along_axis, etc.)
3. Update old CI run
4. Move some stuff from viz/utils.py that really belongs in fixes.py, which is meant to keep version-specific backward compat shims
5. Went through the XXX and TODO in our codebase and took care of some of them, including:
   1. Remove run_tests_as_main
   2. Remove uses of _TempDir in tests in most places
   3. Move morph_maps code to their own file rather than living in surface.py
   4. Remove DigMontage.dev_head_t as it's never used or set, and probably was never meant to be there (we have a warning that this class should not be instantiated by users anyway)
   5. Bump pydata-sphinx-theme to 0.6.3 so we can get rid of our custom template fix

I'm going to update the "make a release" instructions with steps 1-3 since I think it should come right after the deprecations PR (as it is here).

@drammock I left a fun one behind that you can tackle in a follow-up PR in mne/viz/_figure.py:

        # XXX when we support a newer matplotlib (something >3.0) the
        # instructions can have inline bold formatting:
        # instructions = '\n'.join(
        #     [r'$\mathbf{Left‐click~&~drag~on~plot:}$ create/modify annotation',  # noqa E501
        #      r'$\mathbf{Right‐click~on~plot~annotation:}$ delete annotation',
        #      r'$\mathbf{Type~in~annotation~window:}$ modify new label name',
        #      r'$\mathbf{Enter~(or~click~button):}$ add new label to list',
        #      r'$\mathbf{Esc:}$ exit annotation mode & close this window'])

[drammock]

I would keep this pinned. The theme is still under pretty active development, and its test suite is still being refined.

Suggested change

	pydata-sphinx-theme>=0.6.3
	pydata-sphinx-theme==0.6.3

[larsoner]

I assumed that was the original rationale, but I'm hoping that by doing this we can catch their bugs earlier, similar to how we build CircleCI with sphinx-gallery:master. I figure if it starts breaking things in a way we can't easily fix then we can easily add a fixed pin later

[drammock]

fair enough. I guess staying on latest point release is not as dangerous as being on their latest main, anyway.

[agramfort]

what a cleanup !