Fix zillion deprecation warnings in core.dtypes.common
Looking through a recent Travis log I see tons of these:

```
  /home/travis/build/pandas-dev/pandas/pandas/core/dtypes/common.py:2026: DeprecationWarning: elementwise == comparison failed; this will raise an error in the future.
    if dtype in [object, np.object_, 'object', 'O']:
```

This is driven by (probably not desired) cases where `dtype` is array-like at this point (also showed up in #22092).  Easily avoided by checking for hashability before checking for inclusion.
Hello @jbrockmendel! Thanks for updating the PR.



Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on July 31, 2018 at 17:39 Hours UTC this is #22101 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22142?src=pr&el=h1) Report
> Merging [#22142](https://codecov.io/gh/pandas-dev/pandas/pull/22142?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/c272c52a548d4b75d32857d8b33821a3b0b41187?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22142/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/22142?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #22142      +/-   ##
==========================================
- Coverage   92.07%   92.06%   -0.01%     
==========================================
  Files         170      170              
  Lines       50688    50689       +1     
==========================================
- Hits        46671    46669       -2     
- Misses       4017     4020       +3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.47% <100%> (-0.01%)` | :arrow_down: |
| #single | `42.29% <100%> (-0.03%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22142?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/dtypes/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/22142/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbW1vbi5weQ==) | `94.87% <100%> (-0.34%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/22142/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `85.69% <0%> (-0.21%)` | :arrow_down: |
| [pandas/core/arrays/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/22142/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lcy5weQ==) | `95.44% <0%> (-0.03%)` | :arrow_down: |
| [pandas/core/dtypes/dtypes.py](https://codecov.io/gh/pandas-dev/pandas/pull/22142/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2R0eXBlcy5weQ==) | `96.05% <0%> (+0.02%)` | :arrow_up: |
| [pandas/core/indexes/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/22142/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9wZXJpb2QucHk=) | `93.5% <0%> (+0.1%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22142?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/22142?src=pr&el=footer). Last update [c272c52...5d14359](https://codecov.io/gh/pandas-dev/pandas/pull/22142?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!