Repin alembic
Summary: Alembic 1.6.3 broke us, and this has been fixed in 1.6.4. See https://github.com/sqlalchemy/alembic/issues/844 and https://github.com/sqlalchemy/alembic/issues/848.
Test Plan: Unit
Reviewers: dgibson, alangenfeld, prha
Reviewed By: dgibson
Differential Revision: https://dagster.phacility.com/D8093