Fix import-time DeprecationWarning on Python 3.5

[embray]

Fixes the following:

$ python3.5 -Wall
Python 3.5.0 (default, Sep 18 2015, 09:31:01) 
[GCC 4.4.7 20120313 (Red Hat 4.4.7-11)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import pandas.tslib
pandas/__init__.py:7: DeprecationWarning: bad escape \s
  from pandas import hashtable, tslib, lib

In Python 3.5 re.sub replacement patterns containing unrecognized character escapes are deprecated. This raw string format is needed to substitute \s literally.

[jreback]

any idea why this doesn't show the same warning? (this is not in cython)

(py3.5)bash-3.2$ python -Wall
Python 3.5.0 |Continuum Analytics, Inc.| (default, Oct 20 2015, 14:39:26) 
[GCC 4.2.1 (Apple Inc. build 5577)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from re import compile as re_compile
>>> re_compile('\s+')
re.compile('\\s+')

[embray]

@jreback This doesn't affect re.compile() since to it \s is a recognized escape sequence in the context of a regular expression. In a replacement pattern to re.sub, however, it is not a meaningful escape sequence (in the regular expression language), so it needs to be passed literally \\s in order for it to replace with the literal \s.

[jreback]

thanks!