python: declare free-threading support in graph modules

Add pybind11::mod_gil_not_used() to the min_cost_flow, max_flow and
linear_sum_assignment modules so that importing them on a free-threaded
CPython build (3.13t/3.14t) no longer re-enables the GIL.

Author: joshlk

ortools/graph/python/linear_sum_assignment.cc

@@ -20,7 +20,7 @@ using ::operations_research::SimpleLinearSumAssignment;
 using NodeIndex = ::operations_research::SimpleLinearSumAssignment::NodeIndex;
 using ::pybind11::arg;
 
-PYBIND11_MODULE(linear_sum_assignment, m) {
+PYBIND11_MODULE(linear_sum_assignment, m, pybind11::mod_gil_not_used()) {
   pybind11::class_<SimpleLinearSumAssignment> slsa(m,
                                                    "SimpleLinearSumAssignment");
   slsa.def(pybind11::init<>());

ortools/graph/python/max_flow.cc

@@ -22,7 +22,7 @@
 using ::operations_research::SimpleMaxFlow;
 using ::pybind11::arg;
 
-PYBIND11_MODULE(max_flow, m) {
+PYBIND11_MODULE(max_flow, m, pybind11::mod_gil_not_used()) {
   pybind11::class_<SimpleMaxFlow> smf(m, "SimpleMaxFlow");
   smf.def(pybind11::init<>());
   smf.def("add_arc_with_capacity", &SimpleMaxFlow::AddArcWithCapacity,

ortools/graph/python/min_cost_flow.cc

@@ -21,7 +21,7 @@ using ::operations_research::MinCostFlowBase;
 using ::operations_research::SimpleMinCostFlow;
 using ::pybind11::arg;
 
-PYBIND11_MODULE(min_cost_flow, m) {
+PYBIND11_MODULE(min_cost_flow, m, pybind11::mod_gil_not_used()) {
   pybind11::class_<SimpleMinCostFlow> smcf(m, "SimpleMinCostFlow");
   smcf.def(pybind11::init<>());
   smcf.def("add_arc_with_capacity_and_unit_cost",