Override old default values in update_defaults

[djhoese]

See dask/dask#10159

[codecov]

Codecov Report

Merging #63 (5202d7d) into main (c7c736f) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #63      +/-   ##
==========================================
+ Coverage   98.24%   98.28%   +0.03%     
==========================================
  Files           6        6              
  Lines         742      759      +17     
==========================================
+ Hits          729      746      +17     
  Misses         13       13              

Flag	Coverage Δ
unittests	98.28% <100.00%> (+0.03%)	⬆️

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Impacted Files	Coverage Δ
donfig/config_obj.py	95.57% <100.00%> (+0.01%)	⬆️
donfig/tests/test_config.py	100.00% <100.00%> (ø)

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