CLN: small clean-up PeriodIndex (easy parts of #23416)

[jbrockmendel]

xref #23416

[pep8speaks]

Hello @jbrockmendel! Thanks for submitting the PR.

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

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

[codecov]

Codecov Report

Merging #23423 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #23423      +/-   ##
==========================================
+ Coverage   92.19%   92.21%   +0.02%     
==========================================
  Files         161      161              
  Lines       51192    51173      -19     
==========================================
- Hits        47197    47190       -7     
+ Misses       3995     3983      -12

Flag	Coverage Δ
#multiple	90.65% <100%> (+0.02%)	⬆️
#single	42.23% <100%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
pandas/core/arrays/datetimelike.py	96.11% <100%> (+2.1%)	⬆️
pandas/core/indexes/period.py	92.15% <100%> (+1.11%)	⬆️
pandas/io/feather_format.py	77.14% <0%> (-8.58%)	⬇️

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[jorisvandenbossche]

Thanks!