Support Python 3

[nmuesch]

What does this PR do?

Add Python3 support

Motivation

Getting ready for Py3

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

Remove redundant aggregator fixture

[codecov-io]

Codecov Report

Merging #2837 into master will decrease coverage by 0.73%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2837      +/-   ##
==========================================
- Coverage   84.71%   83.97%   -0.74%     
==========================================
  Files         667        4     -663     
  Lines       37912      287   -37625     
  Branches     4550       37    -4513     
==========================================
- Hits        32116      241   -31875     
+ Misses       4462       41    -4421     
+ Partials     1334        5    -1329

[gmmeyer]

This one was very simple. Nice!

[gzussa]

Great!!