Separate qthelp to sphinxcontrib package

[tk0miya]

Feature or Bugfix

• Refactoring

Purpose

• refs: Split some builders and extensions to external package #4971

[codecov]

Codecov Report

Merging #5968 into master will decrease coverage by 0.11%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #5968      +/-   ##
==========================================
- Coverage   82.89%   82.77%   -0.12%     
==========================================
  Files         273      272       -1     
  Lines       39457    39233     -224     
  Branches     5911     5879      -32     
==========================================
- Hits        32706    32475     -231     
- Misses       5385     5395      +10     
+ Partials     1366     1363       -3

Impacted Files	Coverage Δ
sphinx/application.py	77.35% <ø> (ø)	⬆️
tests/test_build.py	97.36% <ø> (ø)	⬆️
setup.py	0% <ø> (ø)	⬆️
sphinx/builders/qthelp.py	0% <0%> (-94.48%)	⬇️
sphinx/util/template.py	90.24% <0%> (-9.76%)	⬇️
sphinx/environment/adapters/indexentries.py	92.55% <0%> (-1.07%)	⬇️
sphinx/writers/html.py	88.22% <0%> (-0.37%)	⬇️

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