Support Python 3

[nmuesch]

What does this PR do?

Adds Python 3 support by changing iteritems, and lightly ensuring unicode in the tests.

Motivation

🐍 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 #3030 into master will decrease coverage by 2.28%.
The diff coverage is 92.3%.

@@            Coverage Diff             @@
##           master    #3030      +/-   ##
==========================================
- Coverage   84.15%   81.86%   -2.29%     
==========================================
  Files         673        4     -669     
  Lines       36595      353   -36242     
  Branches     4336       68    -4268     
==========================================
- Hits        30798      289   -30509     
+ Misses       4548       44    -4504     
+ Partials     1249       20    -1229