 CLN: read_sql date parsing
- [x] closes #15119
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Cleaning up the date parsing portion of the read_sql functionality. Also checking that #15119 can be closed as we are already testing the solution.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24500?src=pr&el=h1) Report
> Merging [#24500](https://codecov.io/gh/pandas-dev/pandas/pull/24500?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/681e75cfe09251be7fa01b5660a8a4281cdc5050?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24500/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24500?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24500   +/-   ##
=======================================
  Coverage    92.3%    92.3%           
=======================================
  Files         166      166           
  Lines       52412    52412           
=======================================
  Hits        48381    48381           
  Misses       4031     4031
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.73% <ø> (ø)` | :arrow_up: |
| #single | `43.05% <ø> (-0.01%)` | :arrow_down: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24500?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/24500?src=pr&el=footer). Last update [681e75c...d84c5c0](https://codecov.io/gh/pandas-dev/pandas/pull/24500?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24500?src=pr&el=h1) Report
> Merging [#24500](https://codecov.io/gh/pandas-dev/pandas/pull/24500?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/681e75cfe09251be7fa01b5660a8a4281cdc5050?src=pr&el=desc) will **decrease** coverage by `60.41%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24500/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24500?src=pr&el=tree)

```diff
@@             Coverage Diff             @@
##           master   #24500       +/-   ##
===========================================
- Coverage    92.3%   31.88%   -60.42%     
===========================================
  Files         166      166               
  Lines       52412    52421        +9     
===========================================
- Hits        48381    16717    -31664     
- Misses       4031    35704    +31673
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `30.29% <ø> (-60.44%)` | :arrow_down: |
| #single | `31.88% <ø> (-11.17%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24500?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/formats/latex.py](https://codecov.io/gh/pandas-dev/pandas/pull/24500/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvbGF0ZXgucHk=) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/core/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/24500/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY2F0ZWdvcmljYWwucHk=) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/io/sas/sas\_constants.py](https://codecov.io/gh/pandas-dev/pandas/pull/24500/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3Nhcy9zYXNfY29uc3RhbnRzLnB5) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/tseries/plotting.py](https://codecov.io/gh/pandas-dev/pandas/pull/24500/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvcGxvdHRpbmcucHk=) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/tseries/converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/24500/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvY29udmVydGVyLnB5) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/io/formats/html.py](https://codecov.io/gh/pandas-dev/pandas/pull/24500/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvaHRtbC5weQ==) | `0% <0%> (-98.65%)` | :arrow_down: |
| [pandas/core/groupby/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/24500/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9jYXRlZ29yaWNhbC5weQ==) | `0% <0%> (-95.46%)` | :arrow_down: |
| [pandas/core/reshape/reshape.py](https://codecov.io/gh/pandas-dev/pandas/pull/24500/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9yZXNoYXBlLnB5) | `8.06% <0%> (-91.51%)` | :arrow_down: |
| [pandas/io/sas/sas7bdat.py](https://codecov.io/gh/pandas-dev/pandas/pull/24500/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3Nhcy9zYXM3YmRhdC5weQ==) | `0% <0%> (-91.17%)` | :arrow_down: |
| [pandas/io/sas/sas\_xport.py](https://codecov.io/gh/pandas-dev/pandas/pull/24500/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3Nhcy9zYXNfeHBvcnQucHk=) | `0% <0%> (-90.15%)` | :arrow_down: |
| ... and [126 more](https://codecov.io/gh/pandas-dev/pandas/pull/24500/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24500?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/24500?src=pr&el=footer). Last update [681e75c...4fee9db](https://codecov.io/gh/pandas-dev/pandas/pull/24500?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 looks fine. is there any user api change here? (meaning do we need a whatsnew note) No API change at all, just reorganization. The linked issue already had an existing test, but the test wasn't as strict as it should have been.  thanks!