resolve segfault in dng observed with Python 3.9

Calling the PythonInterpreter constructor set up some Python objects via
Boost Python before Python was properly initialized. This caused a segfault
in _PyObject_GC_Malloc in Python. Not sure what finally triggered that
segfault. Different Boost version? Different Python version? Different
compiler? Bruce Lee beating up Chuck Norris? https://youtu.be/HFm2L03AetU

modules/gui/src/python_shell/python_interpreter.cc

@@ -66,6 +66,9 @@ PythonInterpreter::~PythonInterpreter()
 
 PythonInterpreter& PythonInterpreter::Instance()
 {
+  if(!Py_IsInitialized()){
+    Py_InitializeEx(1);
+  }
   static PythonInterpreter instance;
   return instance;
 }

modules/gui/src/python_shell/python_interpreter_worker.cc

@@ -20,6 +20,7 @@
 
 #include "python_interpreter_worker.hh"
 #include "python_interpreter.hh"
+#include <iostream>
 
 namespace ost { namespace gui {
 
@@ -55,7 +56,6 @@ PythonInterpreterWorker::PythonInterpreterWorker():
   current_id_(),
   awake_(false)
 {
-  Py_InitializeEx(1);
   parse_expr_cmd_=bp::import("parser").attr("expr");
   main_namespace_ = bp::extract<bp::dict>(bp::import("__main__").attr("__dict__"));
   repr_=bp::import("__main__").attr("__builtins__").attr("repr");