BUG: Concatentation of TZ-aware dataframes

[jreback]

closes #12396
closes #18447

supersedes #19327

[codecov]

Codecov Report

Merging #21014 into master will decrease coverage by <.01%.
The diff coverage is 96.66%.

@@            Coverage Diff             @@
##           master   #21014      +/-   ##
==========================================
- Coverage   91.82%   91.82%   -0.01%     
==========================================
  Files         153      153              
  Lines       49505    49500       -5     
==========================================
- Hits        45460    45455       -5     
  Misses       4045     4045

Flag	Coverage Δ
#multiple	90.22% <96.66%> (-0.01%)	⬇️
#single	41.88% <26.66%> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/core/internals.py	95.59% <100%> (ø)	⬆️
pandas/core/indexes/base.py	96.64% <100%> (-0.01%)	⬇️
pandas/core/dtypes/concat.py	99.17% <96.42%> (-0.01%)	⬇️

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[jreback]

closed via 1dcddba

[jreback]

thanks @paul-mannino