[backport] Fix native WSGI-less HTTP server support under Python 3

[webknjaz]

This is a backport of #1785.

What kind of change does this PR introduce?

• backport
• bug fix
• feature
• docs update
• tests/coverage improvement
• refactoring
• other

What is the related issue number (starting with #)

#1377/#1712/#1784/#1785

What is the current behavior? (You can also link to an open issue here)

It's broken under Python 3

What is the new behavior (if this is a feature change)?

It works under Python 3

Other information:

See #1377

Checklist:

• I think the code is well written
• I wrote good commit messages
• I have squashed related commits together after the changes have been approved
• Unit tests for the changes exist
• Integration tests for the changes exist (if applicable)
• I used the same coding conventions as the rest of the project
• The new code doesn't generate linter offenses
• Documentation reflects the changes
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences

[codecov]

Codecov Report

Merging #1786 into maint/17.x will decrease coverage by 3.19%.
The diff coverage is 100%.

@@              Coverage Diff              @@
##           maint/17.x    #1786     +/-   ##
=============================================
- Coverage       81.43%   78.24%   -3.2%     
=============================================
  Files             104      104             
  Lines           13896    13499    -397     
=============================================
- Hits            11316    10562    -754     
- Misses           2580     2937    +357