fix: add Blob.__str__()

[jennifer-richards]

Based on ietf-tools/mailarchive#4029

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.39%. Comparing base (37888b3) to head (b8b7fb4).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
ietf/blobdb/models.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10284      +/-   ##
==========================================
- Coverage   88.39%   88.39%   -0.01%     
==========================================
  Files         325      325              
  Lines       43453    43457       +4     
==========================================
+ Hits        38410    38413       +3     
- Misses       5043     5044       +1     

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