Use iterator in access token migration

[acidtv]

Fixes #1521

Description of the Change

This PR uses .iterator() instead of .all() in the 0012_add_token_checksum migration to prevent reading the whole table into memory.

I'm not sure if it makes sense to unittest this.

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• CHANGELOG.md updated (only for user relevant changes)
• author name in AUTHORS

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.38%. Comparing base (e34819a) to head (735c3fe).
Report is 12 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1522      +/-   ##
==========================================
- Coverage   97.41%   97.38%   -0.04%     
==========================================
  Files          34       34              
  Lines        2164     2214      +50     
==========================================
+ Hits         2108     2156      +48     
- Misses         56       58       +2     

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[n2ygk]

Thanks!