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/systemc/core/sc_time_python.cc

@@ -36,7 +36,7 @@ namespace
 struct InstallScTime : public ::sc_gem5::PythonInitFunc
 {
     void
-    run(pybind11::module &systemc) override
+    run(pybind11::module_ &systemc) override
     {
         pybind11::class_<sc_core::sc_time> sc_time(systemc, "sc_time");
         sc_time