TST: Finish groupby aggregate cleanup

[alysivji]

Continuation of #18931

• Converted class-based setup fixtures to pytest fixtures
• Consolidated all imports at the top of the file

test_aggregate_item_by_item has an unused DataFrame in the test so cleaned that up as well

Please let me know if you need me to make any changes.

[jreback]

lgtm. ping on green.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@856c92b). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master   #19094   +/-   ##
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  Coverage          ?   91.53%           
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  Files             ?      148           
  Lines             ?    48688           
  Branches          ?        0           
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  Hits              ?    44567           
  Misses            ?     4121           
  Partials          ?        0

Flag	Coverage Δ
#multiple	89.9% <ø> (?)
#single	41.63% <ø> (?)

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[alysivji]

@jreback we are green!

[jreback]

thanks @alysivji nice PR!