PERF: Use ccalendar.get_days_in_month over tslib.monthrange

[jbrockmendel]

asv looks like a wash, probably to be expected since these calls make up such a small part of the methods they are used in.

tslibs.resolution uses the stdlib calendar.monthrange instead of tslib.monthrange, so the perf bump should be bigger there.

[codecov]

Codecov Report

Merging #21451 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #21451   +/-   ##
=======================================
  Coverage   91.89%   91.89%           
=======================================
  Files         153      153           
  Lines       49600    49600           
=======================================
  Hits        45580    45580           
  Misses       4020     4020

Flag	Coverage Δ
#multiple	90.29% <100%> (ø)	⬆️
#single	41.86% <0%> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/tseries/offsets.py	97% <100%> (ø)	⬆️
pandas/core/indexes/datetimes.py	95.8% <100%> (ø)	⬆️

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[gfyoung]

@jbrockmendel : Any asv-related information + whatsnew in 0.24.0 would be good here.

[jreback]

this is fine, no whatsnew needed if just internal refactoring generally (unless its 'big') or has an api impact.