CLN/DOC: Interval and IntervalIndex classes

[jschendel]

Progress towards #16130

• passes git diff upstream/master -u -- "*.py" | flake8 --diff

Changes shouldn't impact the behavior of Interval or IntervalIndex:

• Restructured the IntervalMixin class, as it was using legacy syntax for defining properties (see here)
• Added docstrings to various Interval and IntervalIndex methods and attributes
• Replaced % syntax with .format
• Minor variable renaming for additional clarity, e.g. l -> lhs
  • My linter was complaining PEP 8 (E741): ambiguous variable name 'l'

[codecov]

Codecov Report

Merging #18585 into master will decrease coverage by 0.04%.
The diff coverage is 80.64%.

@@            Coverage Diff             @@
##           master   #18585      +/-   ##
==========================================
- Coverage   91.44%    91.4%   -0.05%     
==========================================
  Files         157      157              
  Lines       51378    51379       +1     
==========================================
- Hits        46985    46964      -21     
- Misses       4393     4415      +22

Flag	Coverage Δ
#multiple	89.27% <80.64%> (-0.03%)	⬇️
#single	40.57% <9.67%> (-0.11%)	⬇️

Impacted Files	Coverage Δ
pandas/core/indexes/interval.py	93.05% <80.64%> (-0.17%)	⬇️
pandas/io/gbq.py	25% <0%> (-58.34%)	⬇️
pandas/plotting/_converter.py	63.44% <0%> (-1.82%)	⬇️
pandas/core/frame.py	97.81% <0%> (-0.1%)	⬇️

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[jreback]

nice!

My linter was complaining PEP 8 (E741): ambiguous variable name 'l'

yeah this is in the newest flake8, we are currently pinning on travis, there is an issue about this, so thanks for changing!