fix: Table name should be quoted by back quotes (`) on DROP TABLE (#385)

Author: ankiaga

google/cloud/sqlalchemy_spanner/sqlalchemy_spanner.py

@@ -438,7 +438,7 @@ def visit_drop_table(self, drop_table, **kw):
         for index in drop_table.element.indexes:
             indexes += "DROP INDEX {};".format(self.preparer.quote(index.name))
 
-        return indexes + constrs + str(drop_table)
+        return indexes + constrs + super().visit_drop_table(drop_table)
 
     def visit_primary_key_constraint(self, constraint, **kw):
         """Build primary key definition.