WIP: Pass missing tz arg in _parsed_string_to_bounds

[jbrockmendel]

The WIP part is finding a test case that fails under the status quo.

[jreback]

generally you can simply test things locally until you are ready to put up a PR, or simply run them on your own travis branch the same.

[jbrockmendel]

I generally do run the tests locally before pushing (and did this time as well). The "WIP" part is that I'd like to find a new test that works with this fix but fails without it.

[codecov]

Codecov Report

Merging #18471 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #18471      +/-   ##
==========================================
- Coverage   91.36%   91.34%   -0.02%     
==========================================
  Files         163      163              
  Lines       49700    49704       +4     
==========================================
- Hits        45407    45402       -5     
- Misses       4293     4302       +9

Flag	Coverage Δ
#multiple	89.14% <100%> (ø)	⬆️
#single	39.66% <0%> (-0.07%)	⬇️

Impacted Files	Coverage Δ
pandas/core/indexes/datetimes.py	95.51% <100%> (+0.01%)	⬆️
pandas/io/gbq.py	25% <0%> (-58.34%)	⬇️
pandas/core/frame.py	97.8% <0%> (-0.1%)	⬇️

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[jreback]

not sure of the point of this PR.