refactor: sorted imports in migration file

[waketzheng]

Description

• Currently imports in migration file:

from tortoise import migrations
from tortoise.migrations import operations as ops
from tortoise import fields
from tortoise.indexes import Index, PartialIndex
from tortoise.migrations.constraints import UniqueConstraint

• After this PR:

from tortoise import fields, migrations
from tortoise.indexes import Index, PartialIndex
from tortoise.migrations import operations as ops
from tortoise.migrations.constraints import UniqueConstraint

Motivation and Context

To avoid migration file changing by ruff check --fix --extend-select=I

How Has This Been Tested?

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added the changelog accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codspeed-hq]

Merging this PR will not alter performance

✅ 24 untouched benchmarks

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_{Comparing waketzheng:sort-imports (0ff989f) with develop (c2ff6a8)}

[seladb]

Please see one comment, otherwise LGTM