API: Prohibit non-numeric dtypes in IntervalIndex
- [X] closes #19016
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

rebase and push again, fixed some hanging by Travis CI # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19022?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@c6166b0`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19022/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/19022?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #19022   +/-   ##
=========================================
  Coverage          ?   91.53%           
=========================================
  Files             ?      148           
  Lines             ?    48814           
  Branches          ?        0           
=========================================
  Hits              ?    44683           
  Misses            ?     4131           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.91% <100%> (?)` | |
| #single | `41.57% <50%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19022?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/interval.py](https://codecov.io/gh/pandas-dev/pandas/pull/19022/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9pbnRlcnZhbC5weQ==) | `92.19% <100%> (ø)` | |
| [pandas/core/dtypes/dtypes.py](https://codecov.io/gh/pandas-dev/pandas/pull/19022/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2R0eXBlcy5weQ==) | `95.31% <100%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19022?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/19022?src=pr&el=footer). Last update [c6166b0...cc22a03](https://codecov.io/gh/pandas-dev/pandas/pull/19022?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @jschendel 