Build-image: Pip installation should support Python3

[anjakefala]

Hello! =)

A component of my website is composed of python3 files and they have dependencies. When I try to pip3 install those dependencies, the deploy logs note that pip3 is not available.

This has been noted in this issue as well: https://github.com/netlify/build-image/issues/240.

I hack around this atm by pulling down get-pip.py and installing it locally with python3, but this does not take advantage of the great caching stuff you do. I think python3-pip should be part of the default image, especially considering that Python 2.7 is being end-of-life'd.

[duckinator]

@anjakefala I figured this out a bit earlier today: you can specify the Python version with runtime.txt or Pipfile (it looks like 2.7, 3.5, and 3.7 are the supported values). If you choose a 3.x version, you get python3 and pip3 executables.

[anjakefala]

Thanks! @duckinator ^^

[anjakefala]

I can confirm that this worked for me. Thanks @duckinator!

[fernaoguerra]

Thanks! I was looking for this. Just to make 100% clear.
Create the runtime.txt** file in the root of the project and fill it out with one of the options:

3.5
3.7

It took me some time to understand it's different than heroku