Add pip dependency support

[nickthecook]

Need to write a project with Python using pyenv to figure out what options are needed for controlling how pip is called, whether to call pip3, whether there are clues in the environment to that ops should look at, etc.

[nickthecook]

Fixed in 1.13.0. There is a pip.command option which lets the caller specify what command to use instead of the default python3 -m pip.