update gunicorn requirement to 20.0.4

[apiontek]

Running paperless on python 3.8 (on ubuntu 20.04) with gunicorn 19.9.0 yields this error:

/usr/lib/python3.8/os.py:1023: RuntimeWarning: line buffering (buffering=1) isn't supported in binary mode, the default buffer size will be used
  return io.open(fd, *args, **kwargs)

This was resolved in gunicorn 20.0 per this issue and doing a pip install -U gunicorn in my paperless venv gets rid of the error message for me.

[Tooa]

Hi, thanks for reporting this issue with a working solution. Are you interested in submitting a PR to fix this issue?

[apiontek]

Done