Reduce code duplication and add documentation

[mih]

Changelog

🏠 Internal

• Reduce code duplication in datalad.support.json_py

[qlty-cloud-legacy]

Code Climate has analyzed commit 093fb81 and detected 0 issues on this pull request.

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[codecov]

Codecov Report

Merging #6398 (093fb81) into master (9309842) will increase coverage by 0.13%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #6398      +/-   ##
==========================================
+ Coverage   89.85%   89.98%   +0.13%     
==========================================
  Files         344      344              
  Lines       43291    43274      -17     
==========================================
+ Hits        38900    38942      +42     
+ Misses       4391     4332      -59     

Impacted Files	Coverage Δ
datalad/support/json_py.py	98.86% <100.00%> (ø)
datalad/support/gitrepo.py	90.37% <0.00%> (+0.16%)	⬆️
datalad/distribution/tests/test_install.py	100.00% <0.00%> (+0.20%)	⬆️
datalad/tests/utils.py	89.39% <0.00%> (+0.21%)	⬆️
datalad/distribution/tests/test_dataset.py	99.70% <0.00%> (+0.29%)	⬆️
datalad/utils.py	85.25% <0.00%> (+0.53%)	⬆️
datalad/tests/test_tests_utils.py	98.91% <0.00%> (+0.54%)	⬆️
datalad/tests/test_utils.py	97.61% <0.00%> (+0.56%)	⬆️
datalad/tests/test_log.py	99.30% <0.00%> (+0.69%)	⬆️
datalad/distribution/tests/test_update.py	99.82% <0.00%> (+0.69%)	⬆️
... and 5 more

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