Python: Don't leak running futures on shutdown

This patch fixes the following assert on shutdown:
"Shutting down while process /testenv/bin/python is running\"\n".

Change-Id: I4c32ead5e4952b69ffc6037739fd417a632eda1a
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/python/pythonplugin.h

@@ -5,6 +5,8 @@
 
 #include <extensionsystem/iplugin.h>
 
+namespace Utils { class FutureSynchronizer; }
+
 namespace Python::Internal {
 
 class PythonPlugin final : public ExtensionSystem::IPlugin
@@ -17,6 +19,7 @@ public:
     ~PythonPlugin() final;
 
     static PythonPlugin *instance();
+    static Utils::FutureSynchronizer *futureSynchronizer();
 
 private:
     void initialize() final;