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

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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 ``` | [Flag](https://app.codecov.io/gh/jazzband/django-oauth-toolkit/pull/1522/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jazzband) | Coverage Δ | | |---|---|---| | [](https://app.codecov.io/gh/jazzband/django-oauth-toolkit/pull/1522/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jazzband) | `?` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jazzband#carryforward-flags-in-the-pull-request-comment) to find out more.

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