Use raw strings to silence deprecation warnings

[DavidCain]

Beginning with Python 3.6, using a backslash literal before a character (where the two do not form an escape sequence) has been deprecated. It's unclear if the author means for there to be two characters - a backslash and then the following character (e.g. '\-') or a single character (e.g. a newline literal '\n'

https://docs.python.org/dev/whatsnew/3.6.html#deprecated-python-behavior

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To silence these deprecation warnings, use a raw string syntax instead.

Note that these strings are exactly equivalent:

$ PYTHONWARNINGS=always python3
>>> old_pattern = '\{\{(?P<m>[a-zA-Z1-9_]+)\}\}'
<stdin>:1: DeprecationWarning: invalid escape sequence \{
>>> new_pattern = r'\{\{(?P<m>[a-zA-Z1-9_]+)\}\}'
>>> new_pattern == old_pattern
True