opentrons-ot3-image: Add python3 pip module so users can install python packages.

[vegano1]

Overview

We want users to be able to install custom python packages using pip, so let's add that module and configure it so it uses the /userfs/var/ directory when installing packages.

Changelog

• Add python3-pip to opentrons-ot3-image recipe
• Add custom python3-pip action in do_install to install a custom pip.conf and .pth file to set install dir
• Add /var/user-packages/usr/bin to PATH env variable so we can access installed executables.

Test

• Make sure pip was installed by running pip3
• Install a python package not included with the system by running pip3 install gspread
• Make sure the package is installed to /var/user-packages/usr/lib/python3.8/site-packages/
• Make sure we can import this package from the Python interpreter
• Make sure we can import this package from a Jupyter Notebook journal

[sfoster1]

ice, looks good to me!