Use promise.get() instead of promise.value in executor

This small change allows for resolvers to return Promises and still get resolved properly using the `SyncExecutor`.

Author: markusdap

graphql/execution/executor.py

@@ -51,7 +51,7 @@ def on_resolve(data):
 
     p = Promise(executor).catch(on_rejected).then(on_resolve)
     context.executor.wait_until_finished()
-    return p.value
+    return p.get()
 
 
 def execute_operation(exe_context, operation, root_value):