Fix crash when using QObject::disconnect method (when multi-threading is enabled)

It seems this is done with a slot call, which gives up the GIL, but the GIL is needed in the destructor of the PythonQtSignalReceiver when a callable target was given.
Fix this by simply using a PythonQtSafeObjectPtr for the _callable.

Author: usiems

src/PythonQtSignalReceiver.h

@@ -94,7 +94,7 @@ class PYTHONQT_EXPORT PythonQtSignalTarget {
   int       _signalId;
   int       _slotId;
   const PythonQtMethodInfo* _methodInfo;
-  PythonQtObjectPtr _callable;
+  PythonQtSafeObjectPtr _callable;
 };
 
 //! base class for signal receivers