Docs: make pip3 installation instructions consistent with Docker and AWS

[ttmc]

The documentation's instructions for installing pip3 had become inconsistent with how we do it in the AWS deployment scripts and the Dockerfile. We now use sudo apt-get install python3-pip rather than:

sudo apt-get install python3-setuptools
sudo easy_install3 pip

[codecov-io]

Current coverage is 90.91% (diff: 100%)

Merging #481 into master will not change coverage

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[vrde]

👍

minor note: if you are working on a virtual env based on Python3, then usually pip and pip3 are the same commands.

[ttmc]

I think that's covered by this sentence: "In the following instructions, we call it pip3 but you may be able to use pip if that refers to the same thing."

I leave it up to them to determine if they refer to the same thing.