Catch UnicodeDecodeError in is_already_migrated

[Mogost]

Fix #593

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 84.23%. Comparing base (ef24fcb) to head (52de15c).
Report is 39 commits behind head on master.

Files with missing lines	Patch %	Lines
constance/migrations/0003_drop_pickle.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #597      +/-   ##
==========================================
- Coverage   89.92%   84.23%   -5.69%     
==========================================
  Files          31       21      -10     
  Lines        1032      736     -296     
  Branches      104      117      +13     
==========================================
- Hits          928      620     -308     
- Misses         69       83      +14     
+ Partials       35       33       -2     

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[Mogost]

@camilonova I have tagged a new version with this change.