PERF: consolidate imports inside parse_time_string

[qwhelan]

This PR adds an asv benchmark for the creation of Period objects directly from integers, and an associated speedup for the same case.

When calling Period._from_ordinal(), the runtime is unfortunately dominated by a pair of import statements needed to pull in the global config settings display.date_dayfirst and display.date_yearfirst. Thus, simply consolidating them leads to a significant speedup:

Benchmarks that have improved:

       before           after         ratio
     [08395af4]       [696b40f1]
     <parse_time_string~1>       <parse_time_string>
-         176±3ms         98.7±3ms     0.56  period.PeriodIndexConstructor.time_from_ints('D')

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

[pep8speaks]

Hello @qwhelan! Thanks for submitting the PR.

• There are no PEP8 issues in the file asv_bench/benchmarks/period.py !

[codecov]

Codecov Report

Merging #24084 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #24084   +/-   ##
=======================================
  Coverage   42.51%   42.51%           
=======================================
  Files         161      161           
  Lines       51689    51689           
=======================================
  Hits        21974    21974           
  Misses      29715    29715

Flag	Coverage Δ
#single	42.51% <ø> (ø)	⬆️

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[codecov]

Codecov Report

Merging #24084 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #24084   +/-   ##
=======================================
  Coverage   92.21%   92.21%           
=======================================
  Files         161      161           
  Lines       51684    51684           
=======================================
  Hits        47658    47658           
  Misses       4026     4026

Flag	Coverage Δ
#multiple	90.61% <ø> (ø)	⬆️
#single	43% <ø> (ø)	⬆️

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[jreback]

can you rebase on master; I think there might be an error in the benchmark (or if not the script should complete green, the CI has been fixed).

[jreback]

can you remove the warnings from asv/benchmarks/frame_methods and re-push. thanks.

[qwhelan]

@jreback I believe the warnings in frame_methods were handled in another PR, so I've rebased again off master.

[jreback]

thanks @qwhelan