CI: Run coverage on as many tests as possible

[effigies]

With the number of catches we have for optional dependencies (and possibly still some Python version checks), and the tests in place to make sure they work, it is a little odd to check the coverage and see them untried. This should help identify, at a glance, which of these checks are genuinely missed.

coverage.py has no dependencies, so installing it in all cases does not complicate our minimal test cases.

[coveralls]

Coverage increased (+0.07%) to 91.908% when pulling bc0f6e7 on ci/max_coverage into 7927c2d on master.

[codecov-io]

Codecov Report

Merging #722 into master will increase coverage by 0.28%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #722      +/-   ##
==========================================
+ Coverage    89.1%   89.39%   +0.28%     
==========================================
  Files          93       93              
  Lines       11469    11476       +7     
  Branches     1992     1992              
==========================================
+ Hits        10220    10259      +39     
+ Misses        908      882      -26     
+ Partials      341      335       -6

Impacted Files	Coverage Δ
nibabel/streamlines/array_sequence.py	100% <100%> (ø)	⬆️
nibabel/nifti1.py	91.52% <0%> (+0.3%)	⬆️
nibabel/dft.py	81.09% <0%> (+0.6%)	⬆️
nibabel/viewers.py	96.06% <0%> (+0.65%)	⬆️
nibabel/__init__.py	93.61% <0%> (+2.12%)	⬆️
nibabel/casting.py	87.28% <0%> (+2.54%)	⬆️
nibabel/openers.py	81.45% <0%> (+3.22%)	⬆️
nibabel/pkg_info.py	79.31% <0%> (+13.79%)	⬆️
nibabel/environment.py	95% <0%> (+20%)	⬆️
nibabel/pydicom_compat.py	100% <0%> (+35%)	⬆️

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