Support Python 3

[nmuesch]

What does this PR do?

Add support for Python3

Motivation

An effort to make all checks support Py2/3

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
• If PR adds a configuration option, it has been added to the configuration file.

Additional Notes

Anything else we should know when reviewing?

[codecov-io]

Codecov Report

Merging #2786 into master will increase coverage by 3.33%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2786      +/-   ##
==========================================
+ Coverage    84.8%   88.13%   +3.33%     
==========================================
  Files         658        6     -652     
  Lines       37540      295   -37245     
  Branches     4504       44    -4460     
==========================================
- Hits        31834      260   -31574     
+ Misses       4391       19    -4372     
+ Partials     1315       16    -1299