TST: move pandas/tests/io/test_date_converters.py to pandas/tests/io/parsers/parse_dates.py
 - [x] closes #15519
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``
 - [x] whatsnew entry

How can I run tests on `parse_dates.py`?

I am getting `ValueError: no option named 'only_slow'` with `item.config.getoption("--only-slow"):`
pls remove the original file (test_date_converts.py)!

ping on green. lgtm. ping on green. thanks! # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15707?src=pr&el=h1) Report
> Merging [#15707](https://codecov.io/gh/pandas-dev/pandas/pull/15707?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/61f6f6333fb7bb2dedf82736aee6c9878382a06f?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.


```diff
@@            Coverage Diff             @@
##           master   #15707      +/-   ##
==========================================
- Coverage   91.03%      91%   -0.03%     
==========================================
  Files         143      143              
  Lines       49411    49396      -15     
==========================================
- Hits        44979    44951      -28     
- Misses       4432     4445      +13
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15707?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/compare/61f6f6333fb7bb2dedf82736aee6c9878382a06f...0b309d34eb8d872715686162f98c2b08af6d1b51?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :x: |
| [pandas/computation/pytables.py](https://codecov.io/gh/pandas-dev/pandas/compare/61f6f6333fb7bb2dedf82736aee6c9878382a06f...0b309d34eb8d872715686162f98c2b08af6d1b51?src=pr&el=tree#diff-cGFuZGFzL2NvbXB1dGF0aW9uL3B5dGFibGVzLnB5) | `90.75% <0%> (-0.85%)` | :x: |
| [pandas/tools/merge.py](https://codecov.io/gh/pandas-dev/pandas/compare/61f6f6333fb7bb2dedf82736aee6c9878382a06f...0b309d34eb8d872715686162f98c2b08af6d1b51?src=pr&el=tree#diff-cGFuZGFzL3Rvb2xzL21lcmdlLnB5) | `93.68% <0%> (-0.34%)` | :x: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/compare/61f6f6333fb7bb2dedf82736aee6c9878382a06f...0b309d34eb8d872715686162f98c2b08af6d1b51?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.87% <0%> (-0.11%)` | :x: |
| [pandas/core/base.py](https://codecov.io/gh/pandas-dev/pandas/compare/61f6f6333fb7bb2dedf82736aee6c9878382a06f...0b309d34eb8d872715686162f98c2b08af6d1b51?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYmFzZS5weQ==) | `95.48% <0%> (-0.03%)` | :x: |
| [pandas/core/groupby.py](https://codecov.io/gh/pandas-dev/pandas/compare/61f6f6333fb7bb2dedf82736aee6c9878382a06f...0b309d34eb8d872715686162f98c2b08af6d1b51?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS5weQ==) | `94.96% <0%> (-0.03%)` | :x: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/compare/61f6f6333fb7bb2dedf82736aee6c9878382a06f...0b309d34eb8d872715686162f98c2b08af6d1b51?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `94.89% <0%> (-0.02%)` | :x: |
| [pandas/indexes/category.py](https://codecov.io/gh/pandas-dev/pandas/compare/61f6f6333fb7bb2dedf82736aee6c9878382a06f...0b309d34eb8d872715686162f98c2b08af6d1b51?src=pr&el=tree#diff-cGFuZGFzL2luZGV4ZXMvY2F0ZWdvcnkucHk=) | `98.28% <0%> (-0.01%)` | :x: |
| [pandas/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/compare/61f6f6333fb7bb2dedf82736aee6c9878382a06f...0b309d34eb8d872715686162f98c2b08af6d1b51?src=pr&el=tree#diff-cGFuZGFzL2luZGV4ZXMvYmFzZS5weQ==) | `96.14% <0%> (-0.01%)` | :x: |
| [pandas/indexes/multi.py](https://codecov.io/gh/pandas-dev/pandas/compare/61f6f6333fb7bb2dedf82736aee6c9878382a06f...0b309d34eb8d872715686162f98c2b08af6d1b51?src=pr&el=tree#diff-cGFuZGFzL2luZGV4ZXMvbXVsdGkucHk=) | `96.59% <0%> (-0.01%)` | :x: |
| ... and [4 more](https://codecov.io/gh/pandas-dev/pandas/pull/15707?src=pr&el=tree-more) | |

------

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> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15707?src=pr&el=footer). Last update [61f6f63...0b309d3](https://codecov.io/gh/pandas-dev/pandas/compare/61f6f6333fb7bb2dedf82736aee6c9878382a06f...0b309d34eb8d872715686162f98c2b08af6d1b51?el=footer&src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).