Fix Python3.6 depreciation warnings

Fix Python3.6 depreciation warnings

We see on Travis and on tox:
/home/travis/build/daxtens/patchwork/patchwork/tests/test_list.py:79: DeprecationWarning: invalid escape sequence \d
  id_re = re.compile('<tr id="patch_row:(\d+)"')
/home/travis/build/daxtens/patchwork/patchwork/tests/test_mail_settings.py:248: DeprecationWarning: invalid escape sequence \s
  form_re_template = ('<form\s+[^>]*action="%(url)s"[^>]*>'
/home/travis/build/daxtens/patchwork/patchwork/tests/test_parser.py:616: DeprecationWarning: invalid escape sequence \
  '\ No newline at end of file'))

All of these are easy to fix: just mark them as raw strings.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Stephen Finucane <stephen@that.guru>