Fatal error, need to update EntityInterface entityType to be static

[jenlampton]

This is the same as backdrop-contrib/flag#14

Core had an API change. Need an update here like this:

          /**
		   * Implements EntityInterface::entityType().
		   *
		   * @since 1.13.0 This method is now static.
		   */
		  public static function entityType() {
		    return 'entity_test';
		  }

[jenlampton]

PR filed: #28

[quicksketch]

This API change is being reverted in core backdrop/backdrop-issues#3809

[jenlampton]

Great, closing this issue. The core change will make this change unnecessary (and problematic)