Fixing 3.6 Escape Sequence Deprecations in tests/io/parser/usecols.py
@jreback [brought up some warnings](https://github.com/pandas-dev/pandas/pull/17310#issuecomment-353402586) on 3.6 that should be fixed by making the regex an r'string'.

Tests pass, happy to fix all the other occurrences, would need to know how to generate these warnings on my local machine though, as running the same pytest command as CI doesn't seem to bring them up for me.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18918?src=pr&el=h1) Report
> Merging [#18918](https://codecov.io/gh/pandas-dev/pandas/pull/18918?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6164da24a7d3f9469fcad8b987c9c7fac1c8dd25?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18918/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/18918?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18918      +/-   ##
==========================================
+ Coverage   91.65%   91.67%   +0.02%     
==========================================
  Files         154      154              
  Lines       51368    51368              
==========================================
+ Hits        47080    47092      +12     
+ Misses       4288     4276      -12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.54% <ø> (+0.02%)` | :arrow_up: |
| #single | `40.84% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18918?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/18918/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `66.95% <0%> (+1.73%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18918?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/18918?src=pr&el=footer). Last update [6164da2...604b2a6](https://codecov.io/gh/pandas-dev/pandas/pull/18918?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 you export this variable to your env, then run them. ``PANDAS_TESTING_MODE="deprecate"``. this make DeprecationWarnings show up (they normally don't).

look on the 3.6 build to see in travis. see here: https://travis-ci.org/pandas-dev/pandas/jobs/320507120 thanks! FYI https://github.com/pandas-dev/pandas/pull/18968

I fixed the rest.