fix crash with Py3 when releasing memory buffer

src/Gui/Document.cpp

@@ -210,6 +210,7 @@ Document::~Document()
         delete it2->second;
 
     // remove the reference from the object
+    Base::PyGILStateLocker lock;
     _pcDocPy->setInvalid();
     _pcDocPy->DecRef();
     delete d;

src/Gui/ViewProvider.cpp

@@ -95,6 +95,7 @@ ViewProvider::ViewProvider()
 ViewProvider::~ViewProvider()
 {
     if (pyViewObject) {
+        Base::PyGILStateLocker lock;
         pyViewObject->setInvalid();
         pyViewObject->DecRef();
     }