Support Python 3

[nmuesch]

What does this PR do?

Check mostly supported py3, just had to update some iteritems and use new imports.

Motivation

Support 🐍3️⃣

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 #3035 into master will decrease coverage by 32.87%.
The diff coverage is 69.23%.

@@             Coverage Diff             @@
##           master    #3035       +/-   ##
===========================================
- Coverage   84.23%   51.35%   -32.88%     
===========================================
  Files         666        5      -661     
  Lines       35945      923    -35022     
  Branches     4266      125     -4141     
===========================================
- Hits        30278      474    -29804     
+ Misses       4422      426     -3996     
+ Partials     1245       23     -1222

[gmmeyer]

wow this looks great! nice job!