Remove _RequireAttrsABCMeta metaclass and replace with simple check

[basnijholt]

This metaclass is messing with the doc-string and function signature in notebooks making it less useful than the benefit it brings.

Description

Please include a summary of the change and which (if so) issue is fixed.

Fixes #(ISSUE_NUMBER_HERE)

Checklist

• Fixed style issues using pre-commit run --all (first install using pip install pre-commit)
• pytest passed

Type of change

Check relevant option(s).

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• (Code) style fix or documentation update
• This change requires a documentation update

[codecov-commenter]

Codecov Report

Merging #409 (2abccc6) into main (815fd31) will decrease coverage by 0.04%.
The diff coverage is 75.00%.

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@@            Coverage Diff             @@
##             main     #409      +/-   ##
==========================================
- Coverage   78.16%   78.13%   -0.04%     
==========================================
  Files          38       38              
  Lines        5529     5534       +5     
  Branches      989      989              
==========================================
+ Hits         4322     4324       +2     
- Misses       1057     1059       +2     
- Partials      150      151       +1     

Impacted Files	Coverage Δ
adaptive/utils.py	61.16% <ø> (-0.05%)	⬇️
adaptive/learner/base_learner.py	80.76% <54.54%> (-7.61%)	⬇️
adaptive/learner/average_learner.py	79.50% <100.00%> (+0.16%)	⬆️
adaptive/learner/average_learner1D.py	74.83% <100.00%> (+0.08%)	⬆️
adaptive/learner/balancing_learner.py	71.87% <100.00%> (+0.12%)	⬆️
adaptive/learner/data_saver.py	77.14% <100.00%> (+0.33%)	⬆️
adaptive/learner/integrator_learner.py	88.56% <100.00%> (+0.03%)	⬆️
adaptive/learner/learner1D.py	87.74% <100.00%> (+0.03%)	⬆️
adaptive/learner/learner2D.py	77.34% <100.00%> (+0.07%)	⬆️
adaptive/learner/learnerND.py	59.73% <100.00%> (+0.07%)	⬆️
... and 1 more

... and 2 files with indirect coverage changes

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