Backwards compatibility for deferrable kwarg

This was previously accepted to the original repository in
ESSolutions/django-mssql-backend#86.

Backwards compatibility for Django < 3.1 is maintained by not directly
trying to load the supports_deferrable_unique_constraints via dot
notation, but rather by getattr with a default.

mssql/schema.py

@@ -689,7 +689,7 @@ def add_field(self, model, field):
             self.connection.close()
 
     def _create_unique_sql(self, model, columns, name=None, condition=None, deferrable=None):
-        if (deferrable and not self.connection.features.supports_deferrable_unique_constraints):
+        if (deferrable and not getattr(self.connection.features, 'supports_deferrable_unique_constraints', False)):
             return None
 
         def create_unique_name(*args, **kwargs):