CLN: Remove PY2/3 references in pandas/util

[gfyoung]

xref #25725.

[codecov]

Codecov Report

Merging #25962 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #25962      +/-   ##
==========================================
- Coverage   91.84%   91.83%   -0.01%     
==========================================
  Files         175      175              
  Lines       52553    52550       -3     
==========================================
- Hits        48269    48261       -8     
- Misses       4284     4289       +5

Flag	Coverage Δ
#multiple	90.39% <100%> (-0.01%)	⬇️
#single	41.9% <100%> (-0.07%)	⬇️

Impacted Files	Coverage Δ
pandas/util/_validators.py	96.8% <ø> (ø)	⬆️
pandas/util/testing.py	90.61% <100%> (-0.13%)	⬇️
pandas/util/_decorators.py	91.42% <100%> (-0.09%)	⬇️
pandas/util/_test_decorators.py	93.22% <100%> (ø)	⬆️
pandas/io/gbq.py	75% <0%> (-12.5%)	⬇️
pandas/core/frame.py	96.79% <0%> (-0.12%)	⬇️

---

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[codecov]

Codecov Report

Merging #25962 into master will decrease coverage by 1.45%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #25962      +/-   ##
==========================================
- Coverage   91.84%   90.39%   -1.46%     
==========================================
  Files         175      175              
  Lines       52553    52550       -3     
==========================================
- Hits        48269    47503     -766     
- Misses       4284     5047     +763

Flag	Coverage Δ
#multiple	90.39% <100%> (-0.01%)	⬇️
#single	?

Impacted Files	Coverage Δ
pandas/util/_validators.py	96.8% <ø> (ø)	⬆️
pandas/util/testing.py	90.61% <100%> (-0.13%)	⬇️
pandas/util/_decorators.py	91.42% <100%> (-0.09%)	⬇️
pandas/util/_test_decorators.py	93.22% <100%> (ø)	⬆️
pandas/io/feather_format.py	21.05% <0%> (-68.43%)	⬇️
pandas/core/computation/pytables.py	62.5% <0%> (-27.75%)	⬇️
pandas/io/pytables.py	63.43% <0%> (-26.8%)	⬇️
pandas/io/gbq.py	75% <0%> (-12.5%)	⬇️
pandas/core/computation/common.py	84.21% <0%> (-5.27%)	⬇️
pandas/core/computation/expr.py	84.78% <0%> (-3.75%)	⬇️
... and 6 more

---

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[jreback]

lgtm. merge on green.