Fix an invalid escape sequence

[yan12125]

Fix errors like this:

$ python -W error -c 'import requests'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/Users/yen/Projects/requests/requests/__init__.py", line 114, in <module>
    from .models import Request, Response, PreparedRequest
  File "/Users/yen/Projects/requests/requests/models.py", line 43, in <module>
    from .status_codes import codes
  File "/Users/yen/Projects/requests/requests/status_codes.py", line 18
    """
SyntaxError: invalid escape sequence \o

Context: the buildbot test suite treats all warnings as errors.

[codecov-io]

Codecov Report

Merging #4728 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master    #4728   +/-   ##
=======================================
  Coverage   66.79%   66.79%           
=======================================
  Files          15       15           
  Lines        1563     1563           
=======================================
  Hits         1044     1044           
  Misses        519      519

Impacted Files	Coverage Δ
requests/status_codes.py	0% <0%> (ø)	⬆️

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