Fix invalid escape sequences

[jwodder]

The code currently contains a number of invalid escape sequences (mostly escapes of special characters in regexes). These result in Python emitting DeprecationWarnings, and some future Python version will instead treat them as syntax errors. This PR fixes them.

[codecov]

Codecov Report

Merging #6293 (77e1d17) into maint (4d554a0) will decrease coverage by 0.04%.
The diff coverage is 98.13%.

@@            Coverage Diff             @@
##            maint    #6293      +/-   ##
==========================================
- Coverage   90.15%   90.11%   -0.05%     
==========================================
  Files         312      312              
  Lines       42272    42262      -10     
==========================================
- Hits        38111    38083      -28     
- Misses       4161     4179      +18     

Impacted Files	Coverage Δ
datalad/tests/test_log.py	99.30% <87.50%> (ø)
datalad/customremotes/tests/test_archives.py	90.00% <92.30%> (+0.59%)	⬆️
datalad/cmdline/tests/test_main.py	83.79% <100.00%> (-1.04%)	⬇️
datalad/customremotes/tests/test_base.py	97.89% <100.00%> (ø)
datalad/customremotes/tests/test_datalad.py	97.87% <100.00%> (ø)
datalad/downloaders/tests/test_http.py	91.05% <100.00%> (+1.88%)	⬆️
datalad/downloaders/tests/test_providers.py	100.00% <100.00%> (ø)
...atalad/interface/tests/test_add_archive_content.py	99.62% <100.00%> (+<0.01%)	⬆️
datalad/metadata/tests/test_search.py	99.42% <100.00%> (ø)
datalad/support/tests/test_captured_exception.py	100.00% <100.00%> (ø)
... and 19 more

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[mih]

Thx!