swev-id: django__django-16662 - group MigrationWriter imports by type

[casey-brooks]

Fixes #518.

Reproduction and observed failure (on django__django-16662)

1. Check out django__django-16662.
2. Run:

   PYTHONPATH=$PWD:$HOME/.nix-profile/lib/python3.11/site-packages DJANGO_SETTINGS_MODULE=tests.test_sqlite python3 - <<'PY'
   import datetime
   import time
   
   from django.db import migrations, models
   from django.db.migrations.writer import MigrationWriter
   
   migration = type(
       "Migration",
       (migrations.Migration,),
       {
           "operations": [
               migrations.CreateModel(
                   "TimedModel",
                   [
                       ("id", models.AutoField(primary_key=True)),
                       ("created_at", models.DateTimeField(default=datetime.datetime.now)),
                       ("duration", models.FloatField(default=time.time)),
                   ],
                   {},
                   (models.Model,),
               )
           ]
       },
   )
   writer = MigrationWriter(migration)
   print(writer.as_string().split("\n\n", 2)[1])
   PY

3. The generated import block is:

   import datetime
   from django.db import migrations, models
   import time
   

4. Save the snippet above into tmp_imports.py and run:

   PYTHONPATH=$HOME/.local/lib/python3.11/site-packages:$HOME/.nix-profile/lib/python3.11/site-packages python3 -m isort --check-only --diff tmp_imports.py

   Output before this change:

   ERROR: /workspace/django/tmp_imports.py Imports are incorrectly sorted and/or formatted.
   --- /workspace/django/tmp_imports.py:before	2025-12-24 17:33:25.490772
   +++ /workspace/django/tmp_imports.py:after	2025-12-24 17:33:28.788401
   @@ -1,4 +1,4 @@
    import datetime
   -from django.db import migrations, models
    import time
   
   +from django.db import migrations, models
   

Summary

• group migration imports into future, plain, and from blocks so plain imports precede from-imports
• retain the combined from django.db import migrations, models special-case handling
• add WriterTests.test_imports_grouped_plain_before_from covering the new ordering

Testing

• PYTHONPATH=$PWD:$HOME/.nix-profile/lib/python3.11/site-packages DJANGO_SETTINGS_MODULE=tests.test_sqlite python3 tests/runtests.py migrations.test_writer --parallel=1 --verbosity=1
• PYTHONPATH=$PWD:$HOME/.nix-profile/lib/python3.11/site-packages flake8 django/db/migrations/writer.py tests/migrations/test_writer.py

[casey-brooks]

Local Verification

• PYTHONPATH=$PWD:$HOME/.nix-profile/lib/python3.11/site-packages DJANGO_SETTINGS_MODULE=tests.test_sqlite python3 tests/runtests.py migrations.test_writer --parallel=1 --verbosity=1 → 53 passed, 0 failed, 0 skipped
• PYTHONPATH=$PWD:$HOME/.nix-profile/lib/python3.11/site-packages flake8 django/db/migrations/writer.py tests/migrations/test_writer.py → lint clean (no issues)

[noa-lucent]

Changes look good. The new import grouping logic matches the requested ordering and the added test covers the regression scenario.