TST: fix fixture for numpy dtypes
- [x] split off from #23982
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

This was one of the things I stumbled over while working on #23982. There's a fixture for all_numpy_dtypes that got introduced in #21899, but it's a lie - it doesn't cover (nearly) all numpy dtypes. It's currently only used in one test, and so I've adapted it here to be able to actually test all numpy dtypes.

Hello @h-vetinari! Thanks for updating the PR.

 - There are no PEP8 issues in the file [`pandas/conftest.py`](https://github.com/h-vetinari/pandas/blob/5b17f2da4daa5b14be39b7dac34442cd31789628/pandas/conftest.py) !

 - There are no PEP8 issues in the file [`pandas/tests/series/test_duplicates.py`](https://github.com/h-vetinari/pandas/blob/5b17f2da4daa5b14be39b7dac34442cd31789628/pandas/tests/series/test_duplicates.py) !



##### Comment last updated on November 29, 2018 at 06:43 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23983?src=pr&el=h1) Report
> Merging [#23983](https://codecov.io/gh/pandas-dev/pandas/pull/23983?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/982c169a286d436481364d2a6c27b355a83472b5?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23983/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23983?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #23983      +/-   ##
==========================================
- Coverage   92.31%    92.3%   -0.01%     
==========================================
  Files         161      161              
  Lines       51513    51551      +38     
==========================================
+ Hits        47554    47585      +31     
- Misses       3959     3966       +7
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.7% <ø> (-0.01%)` | :arrow_down: |
| #single | `42.44% <ø> (+0.01%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23983?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/arrays/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/23983/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3RpbWVkZWx0YXMucHk=) | `95.95% <0%> (-0.31%)` | :arrow_down: |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/23983/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.32% <0%> (-0.17%)` | :arrow_down: |
| [pandas/core/config.py](https://codecov.io/gh/pandas-dev/pandas/pull/23983/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29uZmlnLnB5) | `87.04% <0%> (-0.13%)` | :arrow_down: |
| [pandas/io/sas/sas\_xport.py](https://codecov.io/gh/pandas-dev/pandas/pull/23983/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3Nhcy9zYXNfeHBvcnQucHk=) | `90.14% <0%> (-0.1%)` | :arrow_down: |
| [pandas/io/formats/printing.py](https://codecov.io/gh/pandas-dev/pandas/pull/23983/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvcHJpbnRpbmcucHk=) | `93.01% <0%> (-0.08%)` | :arrow_down: |
| [pandas/plotting/\_core.py](https://codecov.io/gh/pandas-dev/pandas/pull/23983/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb3JlLnB5) | `83.58% <0%> (-0.05%)` | :arrow_down: |
| [pandas/core/computation/align.py](https://codecov.io/gh/pandas-dev/pandas/pull/23983/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tcHV0YXRpb24vYWxpZ24ucHk=) | `97.84% <0%> (-0.05%)` | :arrow_down: |
| [pandas/core/reshape/melt.py](https://codecov.io/gh/pandas-dev/pandas/pull/23983/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9tZWx0LnB5) | `97.5% <0%> (-0.05%)` | :arrow_down: |
| [pandas/core/dtypes/concat.py](https://codecov.io/gh/pandas-dev/pandas/pull/23983/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbmNhdC5weQ==) | `96.63% <0%> (-0.04%)` | :arrow_down: |
| [pandas/io/formats/style.py](https://codecov.io/gh/pandas-dev/pandas/pull/23983/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvc3R5bGUucHk=) | `96.69% <0%> (-0.03%)` | :arrow_down: |
| ... and [11 more](https://codecov.io/gh/pandas-dev/pandas/pull/23983/diff?src=pr&el=tree-more) | |

------

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> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23983?src=pr&el=footer). Last update [982c169...62e8f8a](https://codecov.io/gh/pandas-dev/pandas/pull/23983?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 lgtm. ping on green. @jreback
ping on green thanks