Fix #6803: Disable parallel build on macOS and py38+

[tk0miya]

Feature or Bugfix

• Bugfix

Purpose

• refs: py38: AttributeError: Can't pickle local object 'ParallelTasks.add_task.<locals>.<lambda>' #6803 and test_logging_in_ParallelTasks fails with Python 3.8 (macos x 10.9.5) #6865
• Since python3.8, the default start method of multiprocessing has changed to "spawn" on macOS for security reasons. The implementation for parallel build are strongly coupled with "fork" method. So this disables the feature temporarily (until reimplemented whole of parallel build feature).
• We can switch the start method to "fork" manually. But it is risky choice. So this does not do that.

[codecov]

Codecov Report

Merging #6879 into 2.2.2 will decrease coverage by <.01%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##            2.2.2    #6879      +/-   ##
==========================================
- Coverage   83.75%   83.74%   -0.01%     
==========================================
  Files         271      271              
  Lines       41014    41022       +8     
  Branches     6002     6003       +1     
==========================================
+ Hits        34352    34355       +3     
- Misses       5335     5339       +4     
- Partials     1327     1328       +1

Impacted Files	Coverage Δ
setup.py	0% <ø> (ø)	⬆️
sphinx/util/parallel.py	74.19% <100%> (+0.56%)	⬆️
sphinx/pycode/parser.py	94.95% <100%> (ø)	⬆️
tests/test_util_logging.py	100% <100%> (ø)	⬆️
sphinx/application.py	77.9% <33.33%> (-0.4%)	⬇️
sphinx/highlighting.py	83.63% <0%> (-2.73%)	⬇️

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[tk0miya]

This PR contains #6874 's change.

[jfbu]

LGTM, test behaves as designed and xfails:

tests/test_util_logging.py::test_logging_in_ParallelTasks XFAIL          [100%]

============================== 1 xfailed in 0.47s ==============================

and is marked as passing on Python 3.7 and 3.6 at my mac ox x... (the above via make test)

I also tested behavior with -j4. With Python 3.8:

$ make html O=-j4
Sphinx v2.2.2+/e742af060 en cours d'exécution
For security reason, parallel mode is disabled on macOS and python3.8 and above.  For more details, please read https://github.com/sphinx-doc/sphinx/issues/6803

and the build proceeds in non-parallel mode, whereas
with Python 3.7.5 parallel build proceeds with no warning. Only to confirm!

[tk0miya]

Thank you for reviewing! Merged.