Py3 Support

[stuertz]

I get the following Traceback with py3

Traceback (most recent call last):
  File "/usr/local/lib/python3.8/site-packages/waitress/channel.py", line 356, in service
    task.service()
  File "/usr/local/lib/python3.8/site-packages/waitress/task.py", line 172, in service
    self.execute()
  File "/usr/local/lib/python3.8/site-packages/waitress/task.py", line 440, in execute
    app_iter = self.channel.server.application(environ, start_response)
  File "/usr/local/lib/python3.8/site-packages/requestlogger/__init__.py", line 55, in __call__
    content_length = content_lengths[0] if content_lengths else len(b''.join(retval))
TypeError: sequence item 0: expected a bytes-like object, str found