bpo-30892: Fix _elementtree module initialization (#2647)

Handle getattr(copy, 'deepcopy') error in _elementtree module
initialization.

Author: vstinner

Modules/_elementtree.c

@@ -3990,6 +3990,11 @@ PyInit__elementtree(void)
     st->deepcopy_obj = PyObject_GetAttrString(temp, "deepcopy");
     Py_XDECREF(temp);
 
+    if (st->deepcopy_obj == NULL) {
+        return NULL;
+    }
+
+    assert(!PyErr_Occurred());
     if (!(st->elementpath_obj = PyImport_ImportModule("xml.etree.ElementPath")))
         return NULL;