So far, the email autoresponder fails after a while, raising the
following exception:

  File "/home/bridgedb/virtualenvs/bridgedb/lib/python3.7/site-packages/twisted/internet/posixbase.py", line 614, in _doReadOrWrite
    why = selectable.doRead()
  File "/home/bridgedb/virtualenvs/bridgedb/lib/python3.7/site-packages/twisted/internet/tcp.py", line 243, in doRead
    return self._dataReceived(data)
  File "/home/bridgedb/virtualenvs/bridgedb/lib/python3.7/site-packages/twisted/internet/tcp.py", line 249, in _dataReceived
    rval = self.protocol.dataReceived(data)
  File "/home/bridgedb/virtualenvs/bridgedb/lib/python3.7/site-packages/twisted/protocols/basic.py", line 454, in dataReceived
    self.lineReceived(line)
  File "/home/bridgedb/virtualenvs/bridgedb/lib/python3.7/site-packages/twisted/mail/smtp.py", line 445, in lineReceived
    return getattr(self, 'state_' + self.mode)(line)
  File "/home/bridgedb/virtualenvs/bridgedb/lib/python3.7/site-packages/twisted/mail/smtp.py", line 705, in dataLineReceived
    m.eomReceived() for m in self.__messages
  File "/home/bridgedb/virtualenvs/bridgedb/lib/python3.7/site-packages/twisted/mail/smtp.py", line 705, in <listcomp>
    m.eomReceived() for m in self.__messages
  File "/home/bridgedb/virtualenvs/bridgedb/lib/python3.7/site-packages/bridgedb-0.10.0+11.g4cdd6a61.dirty-py3.7.egg/bridgedb/distributors/email/server.py", line 230, in eomReceived
    self.responder.reply()
  File "/home/bridgedb/virtualenvs/bridgedb/lib/python3.7/site-packages/bridgedb-0.10.0+11.g4cdd6a61.dirty-py3.7.egg/bridgedb/distributors/email/autoresponder.py", line 574, in reply
    response = self.getMailData()
  File "/home/bridgedb/virtualenvs/bridgedb/lib/python3.7/site-packages/bridgedb-0.10.0+11.g4cdd6a61.dirty-py3.7.egg/bridgedb/distributors/email/autoresponder.py", line 392, in getMailData
    client, lang)
  File "/home/bridgedb/virtualenvs/bridgedb/lib/python3.7/site-packages/bridgedb-0.10.0+11.g4cdd6a61.dirty-py3.7.egg/bridgedb/distributors/email/autoresponder.py", line 101, in createResponseBody
    bridges = context.distributor.getBridges(bridgeRequest, interval)
  File "/home/bridgedb/virtualenvs/bridgedb/lib/python3.7/site-packages/bridgedb-0.10.0+11.g4cdd6a61.dirty-py3.7.egg/bridgedb/distributors/email/distributor.py", line 145, in getBridges
    with bridgedb.Storage.getDB() as db:
  File "/home/bridgedb/virtualenvs/bridgedb/lib/python3.7/site-packages/bridgedb-0.10.0+11.g4cdd6a61.dirty-py3.7.egg/bridgedb/Storage.py", line 352, in __enter__
    return next(self.gen)
  File "/home/bridgedb/virtualenvs/bridgedb/lib/python3.7/site-packages/bridgedb-0.10.0+11.g4cdd6a61.dirty-py3.7.egg/bridgedb/Storage.py", line 472, in getDB
    assert _REFCOUNT == 0
builtins.AssertionError:

It's not clear what caused this regression but it may have been
introduced in commit c1a48d1b, as part of our conversion to Python 3.

This fixes <https://bugs.torproject.org/33945>.