Fix changed column descriptions in diffs

[strayge]

Summary

Detect changes in column descriptions.

Bug

On each enum change linked to EnumField aerich generates useless ALTER TABLE.

Steps to reproduce

1. create model with EnumField
2. aerich init-db
3. add field to enum
4. aerich migrate
5. check new migration

Before

async def upgrade(db: BaseDBAsyncClient) -> str:
    return """
        ALTER TABLE "somemodel" ALTER COLUMN "field" TYPE VARCHAR(6) USING "field"::VARCHAR(6);"""

async def downgrade(db: BaseDBAsyncClient) -> str:
    return """
        ALTER TABLE "somemodel" ALTER COLUMN "field" TYPE VARCHAR(6) USING "field"::VARCHAR(6);"""

After

async def upgrade(db: BaseDBAsyncClient) -> str:
    return """
        COMMENT ON COLUMN "somemodel"."field" IS 'VALUE1: VALUE1
VALUE2: VALUE2
VALUE3: VALUE3';"""

async def downgrade(db: BaseDBAsyncClient) -> str:
    return """
        COMMENT ON COLUMN "somemodel"."field" IS 'VALUE1: VALUE1
VALUE2: VALUE2';"""

[long2ice]

Thanks!