Update build to Active Python Releases

[paulboocock]

Currently we build against 2.7, 3.4, 3.5, 3.6.

We should update to the Active Python Releases as listed on https://www.python.org/downloads/

At the time of writing this is:

• 2.7
• 3.5
• 3.6
• 3.7
• 3.8
• 3.9