Old post but i've the same error when using venv and "RIO Framework". I I use the same solution (thank TitanFighter) of replacing version 0.0.14 by 0.0.12 and it's work.
cd your venv directory, activate it. A t prompt :
"pip uninstall python-multipart" (it's remove version 0.0.14 of my main python)
"pip install python-multipart" ==> this install version 0.0.12 who work wih Rio.