Remove ceiling on Pillow; adjust default reader expectations

[SeanLeRoy]

With the release of imageio 2.28.0 on 04/23/2023, main[base-imageio] failed when using imageio's pillow plugin to access images. As a result of this we pinned 2.27.0. In 2.31.0 it seems this issue was resolved, so this adjusts the versions.

Note It seems instead of RGBA imageio now returns RGB for gifs? Unsure if we want to do some more investigation before merging this.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8c097bd) 93.99% compared to head (412c3a1) 93.97%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #543      +/-   ##
==========================================
- Coverage   93.99%   93.97%   -0.03%     
==========================================
  Files          53       53              
  Lines        4662     4662              
==========================================
- Hits         4382     4381       -1     
- Misses        280      281       +1     

Files	Coverage Δ
...tests/readers/extra_readers/test_default_reader.py	100.00% <ø> (ø)
...ts/writers/extra_writers/test_timeseries_writer.py	100.00% <ø> (ø)

... and 1 file with indirect coverage changes

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[toloudis]

LGTM. I don't know if any users will have trouble with the more restrictive constraint here but I don't think this reader is really used all that much. I hope.

Are the unit test changes due to actual changes in the way imageio behaves? I believe GIF supports only binary transparency but not an alpha channel.

[SeanLeRoy]

Are the unit test changes due to actual changes in the way imageio behaves?

Yes, I believe so