WIP: Pass missing tz arg in _parsed_string_to_bounds
The WIP part is finding a test case that fails under the status quo.

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. 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](https://codecov.io/gh/pandas-dev/pandas/pull/18471?src=pr&el=h1) Report
> Merging [#18471](https://codecov.io/gh/pandas-dev/pandas/pull/18471?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/aaee541b538559f8887881ab23d2734dddd920d3?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18471/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/18471?src=pr&el=tree)

```diff
@@            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)` | :arrow_up: |
| #single | `39.66% <0%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18471?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/18471?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.51% <100%> (+0.01%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18471?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18471?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.8% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18471?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18471?src=pr&el=footer). Last update [aaee541...f080bd1](https://codecov.io/gh/pandas-dev/pandas/pull/18471?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 not sure of the point of this PR.