Revert change breaking sqlalchemy 1.4.x (jupyter#1823)

Co-authored-by: Tunç Başar Köse <[email protected]>

nbgrader/api.py

@@ -9,7 +9,7 @@
                         DateTime, Interval, Float, Enum, UniqueConstraint,
                         Boolean, inspect, text)
 from sqlalchemy.orm import (sessionmaker, scoped_session, relationship,
-                            column_property, DeclarativeBase)
+                            column_property, declarative_base)
 from sqlalchemy.orm.exc import NoResultFound, FlushError
 from sqlalchemy.ext.associationproxy import association_proxy
 from sqlalchemy.exc import IntegrityError, StatementError
@@ -25,8 +25,7 @@
 from .auth import Authenticator
 
 
-class Base(DeclarativeBase):
-    pass
+Base = declarative_base()
 
 
 def new_uuid() -> str: