BUG: Fix marker for high memory

[TomAugspurger]

Added strict flag to catch this in the future

xref #18427 (comment)

[codecov]

Codecov Report

Merging #18526 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #18526      +/-   ##
==========================================
- Coverage   91.35%   91.33%   -0.03%     
==========================================
  Files         164      164              
  Lines       49819    49801      -18     
==========================================
- Hits        45512    45485      -27     
- Misses       4307     4316       +9

Flag	Coverage Δ
#multiple	89.13% <ø> (-0.01%)	⬇️
#single	40.81% <ø> (-0.06%)	⬇️

Impacted Files	Coverage Δ
pandas/io/gbq.py	25% <0%> (-58.34%)	⬇️
pandas/core/frame.py	97.81% <0%> (-0.1%)	⬇️
pandas/tseries/offsets.py	96.9% <0%> (-0.05%)	⬇️

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[jreback]

thanks @TomAugspurger