ext: Update pybind11 to version 2.6.2.

This should help reduce warning spew when building with newer compilers.
The pybind11::module type has been renamed pybind11::module_ to avoid
conflicts with c++20 modules, according to the pybind11 changelog, so
this CL also updates gem5 source to use the new type. There is
supposedly an alias pybind11::module which is for compatibility, but we
still get linker errors without changing to pybind11::module_.

Change-Id: I0acb36215b33e3a713866baec43f5af630c356ee
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40255
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

src/sim/python.cc

@@ -33,9 +33,9 @@ namespace
 {
 
 void
-sim_pybind(pybind11::module &m_internal)
+sim_pybind(pybind11::module_ &m_internal)
 {
-    pybind11::module m = m_internal.def_submodule("sim");
+    pybind11::module_ m = m_internal.def_submodule("sim");
     pybind11::class_<
         Port, std::unique_ptr<Port, pybind11::nodelete>>(m, "Port")
         .def("bind", &Port::bind)