Failure after upgrading to Ubuntu 18.04 LTS

Hello,

I've recently upgraded the Ubuntu system hosting the PatchServer from version 16.04 LTS to version 18.04 LTS. The service is now failing to start with the message:
```
$ sudo systemctl status patchserver.service

● patchserver.service - Patch Server daemon
Loaded: loaded (/etc/systemd/system/patchserver.service; enabled; vendor preset: enabled)
Active: failed (Result: exit-code) since Wed 2019-06-12 14:24:17 EDT; 20s ago
Process: 22753 ExecStart=/usr/local/patchserver-venv/bin/gunicorn --pid /run/gunicorn/pid --config /opt/patchserver/config.py --log-level info --access-logfile /opt/patchserver/access.log wsgi (code=exited, status=1/FAILURE)
Main PID: 22753 (code=exited, status=1/FAILURE)

Jun 12 14:24:17 pnt-jamfpro1 gunicorn[22753]: from gunicorn import util
Jun 12 14:24:17 pnt-jamfpro1 gunicorn[22753]: File "/usr/local/patchserver-venv/local/lib/python2.7/site-packages/gunicorn/util.py", line 12, in
Jun 12 14:24:17 pnt-jamfpro1 gunicorn[22753]: import pkg_resources
Jun 12 14:24:17 pnt-jamfpro1 gunicorn[22753]: File "/usr/local/patchserver-venv/local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 71, in
Jun 12 14:24:17 pnt-jamfpro1 gunicorn[22753]: __import__('pkg_resources.extern.packaging.requirements')
Jun 12 14:24:17 pnt-jamfpro1 gunicorn[22753]: File "/usr/local/patchserver-venv/local/lib/python2.7/site-packages/pkg_resources/extern/__init__.py", line 60, in load_module
Jun 12 14:24:17 pnt-jamfpro1 gunicorn[22753]: "distribution.".format(**locals())
Jun 12 14:24:17 pnt-jamfpro1 gunicorn[22753]: ImportError: The 'packaging.requirements' package is required; normally this is bundled with this package so if you get this warning, consult the packager of your distribution.
Jun 12 14:24:17 pnt-jamfpro1 systemd[1]: patchserver.service: Main process exited, code=exited, status=1/FAILURE
Jun 12 14:24:17 pnt-jamfpro1 systemd[1]: patchserver.service: Failed with result 'exit-code'.
```
Any advise on how to fix this issue?
Thank you.