Add isort config to match current convention + run isort via pre-commit (if configured)

[jwodder]

Closes #5922.

[yarikoptic]

awesome -- thank you @jwodder , I will test it out. I wonder -- why patch and not internal label?

[jwodder]

@yarikoptic When I tried to add the "internal" label, it didn't come up. I may have made a typo.

[codecov]

Codecov Report

Merging #5923 (380efc5) into maint (d1a19e2) will decrease coverage by 5.47%.
The diff coverage is n/a.

❗ Current head 380efc5 differs from pull request most recent head f1213ac. Consider uploading reports for the commit f1213ac to get more accurate results

@@            Coverage Diff             @@
##            maint    #5923      +/-   ##
==========================================
- Coverage   90.26%   84.78%   -5.48%     
==========================================
  Files         300      297       -3     
  Lines       42513    42477      -36     
==========================================
- Hits        38373    36014    -2359     
- Misses       4140     6463    +2323     

Impacted Files	Coverage Δ
datalad/support/tests/utils.py	0.00% <0.00%> (-100.00%)	⬇️
datalad/metadata/extractors/tests/test_audio.py	19.35% <0.00%> (-80.65%)	⬇️
datalad/metadata/extractors/xmp.py	12.96% <0.00%> (-79.63%)	⬇️
datalad/metadata/extractors/exif.py	18.75% <0.00%> (-78.13%)	⬇️
datalad/metadata/extractors/image.py	19.35% <0.00%> (-77.42%)	⬇️
datalad/metadata/extractors/audio.py	20.00% <0.00%> (-77.15%)	⬇️
datalad/metadata/extractors/tests/test_exif.py	24.00% <0.00%> (-76.00%)	⬇️
datalad/metadata/extractors/tests/test_xmp.py	26.92% <0.00%> (-73.08%)	⬇️
datalad/metadata/extractors/tests/test_image.py	26.92% <0.00%> (-73.08%)	⬇️
datalad/cmdline/tests/test_helpers.py	38.18% <0.00%> (-61.82%)	⬇️
... and 98 more

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[yarikoptic]

appveyor -- a Mac, As this change doesn't mandate anyone to anything, I will proceed with merge