Covariant MaybePromise<T>

Since Promise<T> is Covariant, this should also be. Solves flow errors when trying to introduce MaybePromise<T> into more places within the executor.

Author: leebyron

src/jsutils/MaybePromise.js

@@ -7,4 +7,4 @@
  * @flow strict
  */
 
-export type MaybePromise<T> = Promise<T> | T;
+export type MaybePromise<+T> = Promise<T> | T;