pip-upgrader: Crashes if pip-upgrader exists in requirements.txt

% pip-upgrader
<snip>
Traceback (most recent call last):
  File "/home/username/.virtualenvs/venv/bin/pip-upgrade", line 11, in <module>
    sys.exit(main())
  File "/home/username/.virtualenvs/venv/lib/python3.5/site-packages/pip_upgrader/cli.py", line 70, in main
    packages, options.get('--use-default-index')).detect_available_upgrades(options)
  File "/home/username/.virtualenvs/venv/lib/python3.5/site-packages/pip_upgrader/packages_status_detector.py", line 126, in detect_available_upgrades
    package_status, reason = self._fetch_index_package_info(package_name, current_version)
  File "/home/username/.virtualenvs/venv/lib/python3.5/site-packages/pip_upgrader/packages_status_detector.py", line 167, in _fetch_index_package_info
    return self._parse_simple_html_package_info(package_name, current_version, response)
  File "/home/username/.virtualenvs/venv/lib/python3.5/site-packages/pip_upgrader/packages_status_detector.py", line 228, in _parse_simple_html_package_info
    latest_version = max([vers for vers in all_versions if not vers.is_prerelease and not vers.is_postrelease])
ValueError: max() arg is an empty sequence

% pip-upgrade --version
1.4.0
% python --version
Python 3.5.3

You might not want to support running pip-upgrader on itself, but I don’t think the app should crash because someone has put it in requirements.txt.