Implement delegate_names to allow decorating delegated attributes

[jbrockmendel]

This PR defines a delegate_names decorator that provides an alternative (and to my taste, much nicer) syntax for pinning delegated attributes to Accessor classes.

Effectively this just moves the call to _add_delegate_accessors from after the class definition to a decorator on the class. I find this much harder to overlook when reading a class definition.

• closes #xxxx
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[pep8speaks]

Hello @jbrockmendel! Thanks for submitting the PR.

• There are no PEP8 issues in the file pandas/core/accessor.py !

• There are no PEP8 issues in the file pandas/core/arrays/categorical.py !

• There are no PEP8 issues in the file pandas/core/indexes/accessors.py !

• There are no PEP8 issues in the file pandas/core/indexes/category.py !

[TomAugspurger]

If you fetch and merge master the CI failures should be fixed.

[codecov]

Codecov Report

Merging #22599 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #22599      +/-   ##
==========================================
+ Coverage   92.04%   92.04%   +<.01%     
==========================================
  Files         169      169              
  Lines       50782    50785       +3     
==========================================
+ Hits        46742    46745       +3     
  Misses       4040     4040

Flag	Coverage Δ
#multiple	90.45% <100%> (ø)	⬆️
#single	42.3% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/core/arrays/categorical.py	95.74% <100%> (ø)	⬆️
pandas/core/indexes/accessors.py	90.09% <100%> (ø)	⬆️
pandas/core/indexes/category.py	97.26% <100%> (-0.02%)	⬇️
pandas/core/accessor.py	98.79% <100%> (+0.07%)	⬆️

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[TomAugspurger]

Agreed this is nicer.

Merging on Monday if no objections.

[jreback]

slightly -0 here as I like the _add_* flow. but ok

[jreback]

thanks @jbrockmendel