Also add error_scope assignment operator to complete the rule-of-3 (follow-on to PR #3870). (#3872)

include/pybind11/detail/common.h

@@ -993,6 +993,7 @@ struct error_scope {
     PyObject *type, *value, *trace;
     error_scope() { PyErr_Fetch(&type, &value, &trace); }
     error_scope(const error_scope &) = delete;
+    error_scope &operator=(const error_scope &) = delete;
     ~error_scope() { PyErr_Restore(type, value, trace); }
 };