MNT: Drop test data from MANIFEST.in

[effigies]

Update setup.cfg option to enable package data specification.

Looks like include_package_data=True has the effect of ignoring the package_data dict.

[codecov]

Codecov Report

Merging #845 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #845   +/-   ##
=======================================
  Coverage   90.09%   90.09%           
=======================================
  Files          98       98           
  Lines       12450    12450           
  Branches     2188     2188           
=======================================
  Hits        11217    11217           
  Misses        883      883           
  Partials      350      350

Impacted Files	Coverage Δ
nibabel/pydicom_compat.py	100% <0%> (ø)	⬆️
nibabel/__init__.py	92.85% <0%> (ø)	⬆️
nibabel/viewers.py	96.14% <0%> (ø)	⬆️

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