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_callbacks.cpp

@@ -117,7 +117,14 @@ TEST_SUBMODULE(callbacks, m) {
         }
     });
 
-    class AbstractBase { public: virtual unsigned int func() = 0; };
+    class AbstractBase {
+    public:
+        // [workaround(intel)] = default does not work here
+        // Defaulting this destructor results in linking errors with the Intel compiler
+        // (in Debug builds only, tested with icpc (ICC) 2021.1 Beta 20200827)
+        virtual ~AbstractBase() {};  // NOLINT(modernize-use-equals-default)
+        virtual unsigned int func() = 0;
+    };
     m.def("func_accepting_func_accepting_base", [](std::function<double(AbstractBase&)>) { });
 
     struct MovableObject {