add benchmarks for timestamp parsing

[jbrockmendel]

There's a reasonable case to be made that these and timeseries.ToDatetime benchmarks belong in the same place. LMK if there's a preference.

Small flake8 fixups got timedelta.py passing, enabled linting for modules starting with "t".

[codecov]

Codecov Report

Merging #18698 into master will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #18698      +/-   ##
==========================================
- Coverage    91.6%   91.56%   -0.05%     
==========================================
  Files         153      153              
  Lines       51272    51272              
==========================================
- Hits        46967    46946      -21     
- Misses       4305     4326      +21

Flag	Coverage Δ
#multiple	89.42% <ø> (-0.03%)	⬇️
#single	40.68% <ø> (-0.11%)	⬇️

Impacted Files	Coverage Δ
pandas/io/gbq.py	25% <0%> (-58.34%)	⬇️
pandas/plotting/_converter.py	64.78% <0%> (-1.74%)	⬇️
pandas/core/frame.py	97.81% <0%> (-0.1%)	⬇️

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[jreback]

thanks!