alembic: Fix compatibility with SQLAlchemy 2.0+.

SQLAlchemy 2.0 changed the way that commits/rollbacks are handled
causing the final `UPDATE` to our `alembic_version_<whatever>` tables
to be rolled back instead of committed.

We now use one connection to determine which
`alembic_version_<whatever>` table to use and another to run the
actual migrations. This prevents the erroneous rollback.

This change is compatible with both SQLAlchemy 1.4 and 2.0.

contrib/ast-db-manage/env.py

@@ -67,6 +67,8 @@ def run_migrations_online():
     and associate a connection with the context.
 
     """
+    script_location = config.get_main_option('script_location')
+
     engine = engine_from_config(
                 config.get_section(config.config_ini_section),
                 prefix='sqlalchemy.',
@@ -74,14 +76,12 @@ def run_migrations_online():
 
     logger.info('Testing for an old alembic_version table.')
 
-    connection = engine.connect()
+    with engine.connect() as connection:
         context.configure(
             connection=connection,
-                target_metadata=target_metadata,
-                version_table='alembic_version'
+            target_metadata=target_metadata
         )
 
-    script_location = config.get_main_option('script_location')
         found = False
         mc = context.get_context()
         current_db_revision = mc.get_current_revision()
@@ -124,6 +124,7 @@ def run_migrations_online():
     this tree but if we still don't have an alembic_version_<tree>
     table, alembic will create it.
     """
+    with engine.connect() as connection:
         context.configure(
             connection=connection,
             target_metadata=target_metadata,
@@ -139,11 +140,8 @@ def run_migrations_online():
             logger.info('Creating new alembic_version_%s table.',
                         script_location)
 
-    try:
         with context.begin_transaction():
             context.run_migrations()
-    finally:
-        connection.close()
 
 
 if context.is_offline_mode():