Replace * imports with explicit imports; remove unused declared const…

[jbrockmendel]

…ants

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

[codecov]

Codecov Report

Merging #17470 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #17470      +/-   ##
==========================================
- Coverage   91.16%   91.14%   -0.02%     
==========================================
  Files         163      163              
  Lines       49590    49590              
==========================================
- Hits        45209    45200       -9     
- Misses       4381     4390       +9

Flag	Coverage Δ
#multiple	88.93% <ø> (ø)	⬆️
#single	40.25% <ø> (-0.07%)	⬇️

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

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[jreback]

thanks!

[jreback]

FYI we don't do too much linting for cython files. If you want to add to the script would be helpful (its ci/lint.sh)

[jbrockmendel]

If you want to add to the script would be helpful (its ci/lint.sh)

I've never had much luck with linting cython. Part of why I like the newer @cython.locals and related decorators is the hope that it can be transitioned to valid python so standard tools can be used. That's still a pipe dream for the time being.