Support Python 3

[nmuesch]

What does this PR do?

This test already supported PY3. Mainly adds py3 testing to tox file, cleanup imports and pin checks_base, and cleans up the test folder.

Motivation

Test on 🐍 3️⃣ and cleanup.

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 #2999 into master will increase coverage by 3.99%.
The diff coverage is 90.9%.

@@            Coverage Diff             @@
##           master    #2999      +/-   ##
==========================================
+ Coverage   85.05%   89.05%   +3.99%     
==========================================
  Files         658        6     -652     
  Lines       35833      137   -35696     
  Branches     4243       24    -4219     
==========================================
- Hits        30478      122   -30356     
+ Misses       4113       12    -4101     
+ Partials     1242        3    -1239