Autogenerate migration still has minor typing issues with mypy

[kshpytsya]

The fix for #1253 looks slightly incorrect as the type for down_revision should be Union[str, Sequence[str], None] and not Union[str, None]. down_revision is a tuple for merge revisions.

[CaselIT]

Hi,

Thanks for reporting. If you can send a PR it would be super! Thanks