I like @Hamada's answer above since it relies on the Firebase CLI:

To solve the issue, simply run pip freeze > requirements.txt then deploy :)

I just wanted to suggest that if you don't want to fully deploy your function (since you are still building it) you can instead use the Firebase CLI to call:

firebase init functions

This might seem weird, since you've already called it once to initialize your functions, but after choosing to Overwrite an existing codebase, this gives you a set of questions to which you carefully answer "No, don't overwrite my existing files" and then it offers to install your dependencies, to which you answer "Yes."

? Would you like to initialize a new codebase, or overwrite an existing one? Overwrite
? What language would you like to use to write Cloud Functions? Python
? File functions/requirements.txt already exists. Overwrite? No? 
? File functions/main.py already exists. Overwrite? No? 
? Do you want to install dependencies now? Yes

This should install the necessary packages (from requirements.txt) into your venv without changing anything else in your project.