Follow up to #17422

[jbrockmendel]

As a follow-up, this performs small touchups in a handful of places.

Implement .pxd file for tslibs.frequencies

cimport get_freq_code in period.pyx

Replace isinstance checks in period.pyx with C-equivalents form util

Remove unused imports from period.pyx

Remove code in tseries.frequencies that can now be imported from tslibs.frequencies

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

[codecov]

Codecov Report

Merging #17472 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #17472      +/-   ##
==========================================
- Coverage   91.16%   91.13%   -0.03%     
==========================================
  Files         163      163              
  Lines       49561    49534      -27     
==========================================
- Hits        45180    45144      -36     
- Misses       4381     4390       +9

Flag	Coverage Δ
#multiple	88.92% <100%> (-0.01%)	⬇️
#single	40.22% <100%> (-0.08%)	⬇️

Impacted Files	Coverage Δ
pandas/tseries/frequencies.py	96.11% <100%> (-0.21%)	⬇️
pandas/io/gbq.py	25% <0%> (-58.34%)	⬇️
pandas/core/frame.py	97.72% <0%> (-0.1%)	⬇️

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[jreback]

thanks