build: add python3-pip to ubuntu.Dockerfiles

[cclauss]

https://pypi.org/project/tap2junit says that tap2junit is compatible with legacy Python but says nothing about Python 3 compatibility. Related to #1674

[mhdawson]

LGTM

[thefourtheye]

cc @nodejs/build

[cclauss]

@refack @rvagg @richardlau Is this PR going in the right direction or should I close it?

[rvagg]

I have no idea @cclauss, you tell me, what does this even do? What does it mean to have both python-pip and python3-pip and to install tap2junit with both? What will that do when it's invoked by different versions of Node? I can merge this and rebuild the images but I'd be doing it completely blind to the consequences and I don't like having to scramble to fix something that's broken because I didn't understand it.

[cclauss]

Python 2 and Python 3 have two separate copies of pip. These are usually called pip2 and pip3 but not always so the safe approach is to do python2 -m pip and python3 -m pip to ensure that you are getting the pip that corresponds to the Python that you want to use.

TL;DR They maintain separate "site-packages" directories so that a module like tap2junit needs to be installed into both if we want that module to be accessible to both Py2 and Py3.

On my machine I get:

$ python2 -m pip --version
pip 19.1.1 from /usr/local/lib/python2.7/site-packages/pip (python 2.7)
$ python3 -m pip --version
pip 19.1.1 from /usr/local/lib/python3.7/site-packages/pip (python 3.7)

If I do import tap2junit in a Python 2 script, I will get the module from the first site-package and if I run the exact same code on a Python 3 interpreter, I will get the module from the second.

We will need to have a similar setup on all boxes where we want to test both Pythons.

[rvagg]

should be deployed across all the containers now