Had a similar issue but the above method didn’t work for me. Clarified it with a rather simpler solution:
(venv) $ pip install --upgrade -r requirements.txt
UPDATE:
This command upgrades all packages that have been explicitly listed in your requirements.txt file.
Your requirements.txt file is just a list of pip install arguments placed in a file. They are used to hold the result from pip freeze for the purpose of achieving repeatable installations. In this case, your requirements.txt file contains a pinned version of everything that was installed when pip freeze was run.