Going from [1], also provide a Python 3 specific pip3 command.

[1] https://www.python.org/dev/peps/pep-0394/

General

• This is a new package and it conforms to the quality requirements

Have the results of the proposed changes been tested?

• I use the packages affected by the proposed changes on a regular basis and confirm this PR works for me
• I generally don't use the affected packages but briefly tested this PR