CLN: remove __metaclass__ from code base

[topper-123]

• xref CLN: remove PY2 #25725 & CLN: remove compat.add_metaclass #26165

Removes uses of __metaclass__ from the code base. After #26165 there was actually only one left.

[codecov]

Codecov Report

Merging #26178 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #26178      +/-   ##
==========================================
- Coverage   91.98%   91.98%   -0.01%     
==========================================
  Files         175      175              
  Lines       52371    52370       -1     
==========================================
- Hits        48175    48170       -5     
- Misses       4196     4200       +4

Flag	Coverage Δ
#multiple	90.53% <100%> (-0.01%)	⬇️
#single	40.71% <100%> (-0.15%)	⬇️

Impacted Files	Coverage Δ
pandas/core/computation/engines.py	88.33% <100%> (-0.2%)	⬇️
pandas/io/gbq.py	78.94% <0%> (-10.53%)	⬇️
pandas/core/frame.py	96.9% <0%> (-0.12%)	⬇️

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[WillAyd]

Thanks @topper-123