BUG #19860 Corrected use of mixed indexes with .at
`.at` incorrectly disallowed the use of integer indexes when a mixed index was used
```
s = Series([1,2,3,4,5],index=['a','b','c',1,2])
s.at['a'] # returns 1
s.at[1]   # returns 4
s.at[4]   # raises KeyError, doesn't do fallback indexing
```

Made sure fallback indexing doesn't happen on mixed indexes

- [X] closes #19860
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22436?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@fa47b8d`). [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/22436/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22436?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #22436   +/-   ##
=========================================
  Coverage          ?   92.03%           
=========================================
  Files             ?      169           
  Lines             ?    50780           
  Branches          ?        0           
=========================================
  Hits              ?    46737           
  Misses            ?     4043           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.44% <100%> (?)` | |
| #single | `42.22% <33.33%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22436?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexing.py](https://codecov.io/gh/pandas-dev/pandas/pull/22436/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhpbmcucHk=) | `93.79% <100%> (ø)` | |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/22436/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.45% <100%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22436?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/22436?src=pr&el=footer). Last update [fa47b8d...fcb1f61](https://codecov.io/gh/pandas-dev/pandas/pull/22436?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 also there *might* be other .at issues which this closes. pls have a look if you can.

cc @toobaz  Hello @mariuspot! Thanks for updating the PR.









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

##### Comment last updated on August 27, 2018 at 15:41 Hours UTC looks good. can you rebase looks like you picked up some other commits, do this
``git merge upstream/master`` and push again `git merge upstream/master` didn't do anything
Not sure what I did now :/
Hope it's ok now Ok, I figured out what went wrong with the merge. Should be ok now lgtm ping on green. Looks like travis failed with a segfault on the linker @jreback The travis build had a segfault with the linker while setting up the environment. What is the process for that. Is there a way for me to rerun the build or should I make a change and do a new commit? thanks!