REF: No need to delegate to index check of whether an int is an int
- [x] tests passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Just fixing the following nonsensical error:

``` python
In [2]: pd.Series(range(10)).iloc['a']
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-2-4a32011255fe> in <module>()
----> 1 pd.Series(range(10)).iloc['a']
[...]
TypeError: cannot do positional indexing on <class 'pandas.core.indexes.range.RangeIndex'> with these indexers [a] of <class 'str'>
```

(you just cannot do positional indexing with a ``str``, regardless of the index)

... and adding a docstring while I was at it.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21982?src=pr&el=h1) Report
> Merging [#21982](https://codecov.io/gh/pandas-dev/pandas/pull/21982?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/b9c17861ab994d0ba3b379af7939ecbb6839a862?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/21982/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/21982?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21982      +/-   ##
==========================================
- Coverage   91.96%   91.96%   -0.01%     
==========================================
  Files         166      166              
  Lines       50334    50333       -1     
==========================================
- Hits        46292    46291       -1     
  Misses       4042     4042
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.36% <ø> (-0.01%)` | :arrow_down: |
| #single | `42.21% <ø> (-0.02%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21982?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexing.py](https://codecov.io/gh/pandas-dev/pandas/pull/21982/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhpbmcucHk=) | `93.82% <ø> (+0.08%)` | :arrow_up: |
| [pandas/core/indexes/interval.py](https://codecov.io/gh/pandas-dev/pandas/pull/21982/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9pbnRlcnZhbC5weQ==) | `94.12% <0%> (-0.19%)` | :arrow_down: |
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/21982/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `97.15% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21982?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/21982?src=pr&el=footer). Last update [b9c1786...34cdc8e](https://codecov.io/gh/pandas-dev/pandas/pull/21982?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 See #21989 for the more general issue. Ready to merge if there are no objections. Hello @toobaz! Thanks for updating the PR.







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

##### Comment last updated on July 20, 2018 at 13:23 Hours UTC lgtm. merge on green.