Fixes DeprecationWarnings in Python 3.6 due to invalid escape sequences

Fixes #3954

# [Codecov](https://codecov.io/gh/kennethreitz/requests/pull/3956?src=pr&el=h1) Report
> Merging [#3956](https://codecov.io/gh/kennethreitz/requests/pull/3956?src=pr&el=desc) into [master](https://codecov.io/gh/kennethreitz/requests/commit/342096a3b98e9c4466316167574dbf5054f30daa?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/kennethreitz/requests/pull/3956/graphs/tree.svg?width=650&height=150&src=pr&token=mZGswIiXLa)](https://codecov.io/gh/kennethreitz/requests/pull/3956?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #3956   +/-   ##
=======================================
  Coverage   89.61%   89.61%           
=======================================
  Files          15       15           
  Lines        1946     1946           
=======================================
  Hits         1744     1744           
  Misses        202      202
```


| [Impacted Files](https://codecov.io/gh/kennethreitz/requests/pull/3956?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [requests/models.py](https://codecov.io/gh/kennethreitz/requests/pull/3956?src=pr&el=tree#diff-cmVxdWVzdHMvbW9kZWxzLnB5) | `93.42% <ø> (ø)` | :arrow_up: |
| [requests/api.py](https://codecov.io/gh/kennethreitz/requests/pull/3956?src=pr&el=tree#diff-cmVxdWVzdHMvYXBpLnB5) | `81.81% <ø> (ø)` | :arrow_up: |
| [requests/sessions.py](https://codecov.io/gh/kennethreitz/requests/pull/3956?src=pr&el=tree#diff-cmVxdWVzdHMvc2Vzc2lvbnMucHk=) | `92.93% <ø> (ø)` | :arrow_up: |

------

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> Using raw strings for docstrings tends to surprise people.

How so?

> My experience has been that people don't really grasp that you can prefix the string literal for docstrings, so they expect them to be plain strings.

My (admittedly anecdotal) experience has been with tools that suggest using raw-strings for this exact use-case (e.g., pydocstyle, etc.). *shrug* I don't particularly care either way.

I personally prefer raw strings, they remove any ambiguity and make it clear that escape sequences do not work inside the string. They also remove noise which makes them easier to read. But I don't really care either.

Should I squash this commits or is it fine this way?

Squashing the commits might be nice.

I'm not partial to either approach but as a data point [PEP257](https://www.python.org/dev/peps/pep-0257/#what-is-a-docstring) seems to suggest raw docstrings are the way to go in the case of backslashes.

> Use r"""raw triple double quotes""" if you use any backslashes in your docstrings.

Hrm. I'll accept that as a preponderance of evidence that I'm an idiot then. ;) I'm outvoted 2-to-1, so back to raw strings we go. Sorry @rubendura!

No worries. I'll revert that now.

Done. Should be ready now :)

Yup, looks good. Thanks @rubendura!