PythonProxyNotificationHandler::OnEventGenericHook acquires the Python GIL before executing (#3904)

- Protected python execution of OnEventGenericHook by trying to lock the GIL and show a descriptive error when it was not possible to lock.
- Improved mechanism to lock python mutex and GIL in PhytonSystemComponent. Acquiring/releasing GIL once per thread.
- Added unit test to verify errors are fired when trying to execute OnEventGenericHook from another thread (as it should not able to acquire the GIL).
- Improved python threading tests to actually using python buses.

Signed-off-by: moraaar moraaar@amazon.com

Gems/EditorPythonBindings/Code/Tests/EditorPythonBindingsTest.cpp

@@ -65,7 +65,7 @@ namespace UnitTest
         {
             // clearing up memory
             m_notificationSink = EditorPythonBindingsNotificationBusSink();
-            m_testSink = PythonTraceMessageSink();
+            m_testSink.CleanUp();
 
             // shutdown time!
             PythonTestingFixture::TearDown();
@@ -333,7 +333,7 @@ sys.version
         {
             // clearing up memory
             m_notificationSink = EditorPythonBindingsNotificationBusSink();
-            m_testSink = PythonTraceMessageSink();
+            m_testSink.CleanUp();
 
             // shutdown time!
             PythonTestingFixture::TearDown();