MAINT: Effect FutureWarnings, including deprecations

[effigies]

We've been threatening changes for a while, so we can go ahead and effect some.

I'll have another look through for FutureWarnings to make sure they're appropriate.

The newish deprecation machinery transitions to an error on use when the until version is hit, which means we should keep those in a bit longer to ensure people see the error messages. Some of the things I've moved to that machinery promised removal at 3.0, so I'll also make sure we have tests that will fail if they still exist in the release branch.

Closes #524.

[codecov-io]

Codecov Report

Merging #755 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #755      +/-   ##
==========================================
+ Coverage   88.38%   88.42%   +0.04%     
==========================================
  Files         188      188              
  Lines       24008    23992      -16     
  Branches     4254     4252       -2     
==========================================
- Hits        21219    21215       -4     
+ Misses       2102     2092      -10     
+ Partials      687      685       -2

Impacted Files	Coverage Δ
nibabel/testing/__init__.py	97.89% <100%> (+1.08%)	⬆️
nibabel/__init__.py	93.61% <100%> (ø)	⬆️
nibabel/trackvis.py	94.4% <100%> (+1.33%)	⬆️
nibabel/minc.py	100% <100%> (ø)	⬆️
nibabel/minc1.py	90.36% <100%> (+0.11%)	⬆️
nibabel/checkwarns.py	100% <100%> (ø)	⬆️
venv/Lib/site-packages/nibabel/checkwarns.py	100% <0%> (ø)	⬆️
venv/Lib/site-packages/nibabel/minc1.py	90.36% <0%> (+0.11%)	⬆️
venv/Lib/site-packages/nibabel/testing/__init__.py	97.89% <0%> (+1.08%)	⬆️
... and 1 more

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[effigies]

Only pre-test failing (#768), so this is ready for review.

I think nibabel.trackvis are the changes most warranting careful attention.

I've also added a test file to schedule removal of modules and other API elements. I'd considered trying something a bit more automatic, e.g., deprecate_with_version producing appropriate tests, but don't have the energy for that at the moment. A single place to record intent to deprecate I think is sufficient for now.