BUG: Series[EA].astype(str) works

[TomAugspurger]

Closes #20578

cc @jorisvandenbossche

[codecov]

Codecov Report

Merging #20581 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #20581      +/-   ##
==========================================
+ Coverage   91.82%   91.82%   +<.01%     
==========================================
  Files         152      152              
  Lines       49255    49255              
==========================================
+ Hits        45226    45228       +2     
+ Misses       4029     4027       -2

Flag	Coverage Δ
#multiple	90.21% <100%> (ø)	⬆️
#single	41.9% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/core/internals.py	95.53% <100%> (ø)	⬆️
pandas/util/testing.py	84.73% <0%> (+0.2%)	⬆️

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[jorisvandenbossche]

Thanks!