Add Python3 Support

[nmuesch]

What does this PR do?

Add support for Python3

Motivation

Be compat with py27 and py36

Review checklist

• PR has a meaningful title or PR has the no-changelog label attached
• Feature or bugfix has tests
• Git history is clean
• If PR impacts documentation, docs team has been notified or an issue has been opened on the documentation repo

Additional Notes

Anything else we should know when reviewing?

[codecov-io]

Codecov Report

Merging #2583 into master will decrease coverage by 1.31%.
The diff coverage is 75%.

@@            Coverage Diff            @@
##           master   #2583      +/-   ##
=========================================
- Coverage   83.92%   82.6%   -1.32%     
=========================================
  Files         644       6     -638     
  Lines       36394     184   -36210     
  Branches     4420      25    -4395     
=========================================
- Hits        30544     152   -30392     
+ Misses       4540      18    -4522     
+ Partials     1310      14    -1296