[PATCH] python: make some docstrings raw

[David Bremner]

Jakub Wilk <jwilk at jwilk.net> writes:

> Fixes:
>
>     notmuch/message.py:57: DeprecationWarning: invalid escape sequence \s
>     notmuch/query.py:155: DeprecationWarning: invalid escape sequence \.
>     notmuch/messages.py:89: DeprecationWarning: invalid escape sequence \s
>
> with Python >= 3.6.

Pushed to master. I struggled a bit to understand what those backslashes
are doing there, but they've been there forever...