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/python/m5/SimObject.py

@@ -731,9 +731,9 @@ class MetaSimObject(type):
         code('''namespace py = pybind11;
 
 static void
-module_init(py::module &m_internal)
+module_init(py::module_ &m_internal)
 {
-    py::module m = m_internal.def_submodule("param_${cls}");
+    py::module_ m = m_internal.def_submodule("param_${cls}");
 ''')
         code.indent()
         if cls._base: