FutureWarning: Possible nested set at position 29 with Python 3.7

[mundschenk-at]

When running on Python 3.7, the logparser cron job generates a FutureWarning:

<path>/modoboa_stats/management/commands/logparser.py:100: FutureWarning: Possible nested set at position 29
  self._regex = {k: re.compile(v) for k, v in self._regex.items()}