Support Python 3

[coignetp]

What does this PR do?

Supporting python3 for dns_check. The package dnspython v1.16 we use should support python 3.

Motivation

Additional Notes

It has already been try in #2733

Review checklist (to be filled by reviewers)

• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached
• Feature or bugfix must have tests
• Git history must be clean
• If PR adds a configuration option, it must be added to the configuration file.

[codecov]

Codecov Report

Merging #3425 into master will increase coverage by 5.1%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #3425     +/-   ##
=========================================
+ Coverage   79.04%   84.15%   +5.1%     
=========================================
  Files         155       70     -85     
  Lines        7789     4917   -2872     
  Branches      948      598    -350     
=========================================
- Hits         6157     4138   -2019     
+ Misses       1409      650    -759     
+ Partials      223      129     -94

[albertvaka]

The strings with numbered placeholders (eg: "{0}") should still work with format in python3, so I don't see the need to replace them. In any case, it's not a problem since we don't need to translate these strings. LGTM.