Refactor add_attrs into classes verify()

[aaronspring]

Description

• verify()
• bootstrap()
• now all keywords are captured in skill.attrs
• fix doctests with https://github.com/max-sixty/pytest-accept

Closes #475

To-Do List

Type of change

• Refactoring

How Has This Been Tested?

• Tests added for pytest, if necessary.

Checklist (while developing)

• CHANGELOG is updated with reference to this PR.

References

[codecov]

Codecov Report

Merging #694 (4ba7274) into main (a1b80a2) will decrease coverage by 0.01%.
The diff coverage is 98.13%.

❗ Current head 4ba7274 differs from pull request most recent head 60fa5ec. Consider uploading reports for the commit 60fa5ec to get more accurate results

@@            Coverage Diff             @@
##             main     #694      +/-   ##
==========================================
- Coverage   93.87%   93.85%   -0.02%     
==========================================
  Files          58       58              
  Lines        6250     6214      -36     
==========================================
- Hits         5867     5832      -35     
+ Misses        383      382       -1     

Impacted Files	Coverage Δ
climpred/options.py	81.48% <ø> (ø)
climpred/prediction.py	93.16% <ø> (-0.23%)	⬇️
climpred/reference.py	97.14% <ø> (-0.08%)	⬇️
climpred/utils.py	91.11% <94.11%> (+1.03%)	⬆️
climpred/bootstrap.py	91.66% <100.00%> (-0.27%)	⬇️
climpred/classes.py	94.51% <100.00%> (-0.15%)	⬇️
climpred/metrics.py	91.78% <100.00%> (ø)
climpred/tests/test_HindcastEnsemble_class.py	100.00% <100.00%> (ø)
climpred/tests/test_graphics.py	100.00% <100.00%> (ø)
climpred/tests/test_utils.py	99.28% <100.00%> (+0.58%)	⬆️
... and 2 more

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