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>

ext/pybind11/tests/test_async.cpp

@@ -18,7 +18,7 @@ TEST_SUBMODULE(async_module, m) {
         .def(py::init<>())
         .def("__await__", [](const SupportsAsync& self) -> py::object {
             static_cast<void>(self);
-            py::object loop = py::module::import("asyncio.events").attr("get_event_loop")();
+            py::object loop = py::module_::import("asyncio.events").attr("get_event_loop")();
             py::object f = loop.attr("create_future")();
             f.attr("set_result")(5);
             return f.attr("__await__")();