GHA: Update setup-python version

[jsiirola]

Fixes # .

Summary/Motivation:

This should suppress GHA warnings about using "set-output".

Changes proposed in this PR:

• Update setup-python from @v3 to @v4

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1. I agree my contributions are submitted under the BSD license.
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[codecov]

Codecov Report

Base: 87.04% // Head: 87.04% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (fc8d216) compared to base (517060e).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2705   +/-   ##
=======================================
  Coverage   87.04%   87.04%           
=======================================
  Files         757      757           
  Lines       84596    84596           
=======================================
+ Hits        73638    73639    +1     
+ Misses      10958    10957    -1     

Flag	Coverage Δ
linux	84.45% <ø> (ø)
osx	74.50% <ø> (ø)
other	84.63% <ø> (ø)
win	81.79% <ø> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pyomo/core/expr/visitor.py	93.40% <0.00%> (+0.15%)	⬆️

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