How to cache Python dependencies properly

I am trying to build a Python project in a Dockerfile. I want to cache dependencies, and then use that cache later, something like this:

RUN pip3 download -d "/pth/to/downloaded/files" -r /temp/requirements.txt -c /temp/constraints.txt

# much later on in the Dockerfile:

RUN pip3 install --download-cache="/pth/to/downloaded/files" -r requirements.txt -c constraints.txt

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Question: Assuming the pip3 download command is correct, I no longer see a --download-cache option when I look at the pip3 --help output - is there a new option I can use with pip3 install to reference the dependency cache generated by pip3 download?

Right now I am getting this error:

Usage: pip install [options]

no such option: --download-cache

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