create special rendering for Variant #131
Description
Migrated issue, originally created by Michael Bayer (@zzzeek)
a Variant() type would ideally be rendered like this:
op.add_column(Column("mycol", Variant(Enum, {"mysql": mysql.ENUM(...), "postgresql": postgresql.ENUM(...)})
the special thing here is that Variant() will typically have lots of dialect-specific types in it, which means we need to get dialect prefixes into the render. So this kind of takes it outside the realm of straight repr() and I think alembic should have a special renderer for it.