Upgrade gunicorn

[BenSturmfels]

This avoids "ModuleNotFoundError: No module named 'gunicorn.six.moves'" as seen with gunicorn 19 on Ubuntu 24.04.

[BenSturmfels]

Here's the full traceback:

Traceback (most recent call last):
  File "/home/vhosts/.../env/bin/gunicorn", line 5, in <module>
    from gunicorn.app.wsgiapp import run
  File "/home/vhosts/.../env/lib/python3.12/site-packages/gunicorn/app/wsgiapp.py", line 9, in <module>
    from gunicorn.app.base import Application
  File "/home/vhosts/.../env/lib/python3.12/site-packages/gunicorn/app/base.py", line 11, in <module>
    from gunicorn._compat import execfile_
  File "/home/vhosts/.../env/lib/python3.12/site-packages/gunicorn/_compat.py", line 264, in <module>
    from gunicorn.six.moves.urllib.parse import urlsplit
ModuleNotFoundError: No module named 'gunicorn.six.moves'