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/python.hh

@@ -48,7 +48,7 @@ struct PythonInitFunc
 
     PythonInitFunc();
     ~PythonInitFunc() {}
-    virtual void run(pybind11::module &systemc) = 0;
+    virtual void run(pybind11::module_ &systemc) = 0;
 };
 
 } // namespace sc_gem5