Remove cap on oslo.db version

This commit removes the requirements cap on oslo.db. This was
originally added because of the removal of a private interface that
subunit2sql depended on in it's alembic env.py. A public interface
was eventually added to provide analogous functionality but
subunit2sql never removed the cap and used the new interface;
until now.

Change-Id: Idbc17096d0d3fe43261026b866041ae8a97d86fc

requirements.txt

@@ -1,6 +1,6 @@
 alembic>=0.4.1
 oslo.config>=1.4.0.0a3
-oslo.db!=1.12.0,<2.0.0
+oslo.db>=2.1.0
 oslo.concurrency
 pbr>=1.0.0
 python-subunit>=0.0.18

subunit2sql/migrations/env.py

@@ -79,7 +79,7 @@ def run_migrations_online():
     facade = db_api._create_facade_lazily()
     engine = facade.get_engine()
     connection = engine.connect()
-    facade._session_maker.configure(bind=connection)
+    facade.get_sessionmaker().configure(bind=connection)
 
     context.configure(connection=connection,
                       target_metadata=target_metadata,
@@ -90,7 +90,7 @@ def run_migrations_online():
             context.run_migrations()
     finally:
         connection.close()
-        facade._session_maker.configure(bind=engine)
+        facade.get_sessionmaker().configure(bind=engine)
 
 if context.is_offline_mode():
     run_migrations_offline()

subunit2sql/tests/migrations/test_model_sync.py

@@ -108,7 +108,7 @@ class TestModelsMigrations(test_migrations.ModelsMigrationsSync,
             object_, name, type_, reflected, compare_to)
 
     def filter_metadata_diff(self, diff):
-        return filter(self.remove_unrelated_errors, diff)
+        return list(filter(self.remove_unrelated_errors, diff))
 
     def remove_unrelated_errors(self, element):
         insp = sqlalchemy.engine.reflection.Inspector.from_engine(