fix to migrate2newschema

[djarecka]

using deepcopy(data) in migrate2newschema, so it doesn't change the original data
closes #514

[codecov]

Codecov Report

Merging #515 (e781e29) into master (8f9bff1) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #515      +/-   ##
==========================================
+ Coverage   82.64%   82.65%   +0.01%     
==========================================
  Files          59       59              
  Lines        6107     6112       +5     
==========================================
+ Hits         5047     5052       +5     
  Misses       1060     1060              

Flag	Coverage Δ
unittests	82.65% <100.00%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
dandi/metadata.py	77.48% <100.00%> (+0.05%)	⬆️
dandi/tests/test_metadata.py	100.00% <100.00%> (ø)

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[yarikoptic]

looks kosher - good to merge if tests remain green

[yarikoptic]

Thank you @djarecka !

[yarikoptic]

Damn forgot to add a label for auto