Replace PYBIND11_OVERRIDE_IMPL with PYBIND11_OVERRIDE_NAME

`PYBIND11_OVERRIDE_IMPL` is an implementation detail and not meant to be used outside the pybind11 source tree.

Notes:

* For easy reference: `PYBIND11_OVERRIDE_IMPL` was added to python_games.cc in cl/379240506.

* Discovered in connection with google/pybind11clif#30015, which changes `PYBIND11_OVERRIDE_IMPL`. (The pywrapcc fork of pybind11 is not currently used in production, but we're using it regularly to run TAP Global Presubmits, to test go/pyclif_pybind11_fusion developments.)

* FYI: Test coverage seems to be incomplete: control flow passes through the changed code for many tests, but replacing `"mean_field_population"` with `"XXXmean_field_population"` does not break any tests (see isolated TGP results under http://tap/OCL:517999370:BASE:517998831:1679332509158:a5f7412a). I.e. a test with a Python override is missing.

* FYI: The `PYBIND11_OVERLOAD_*` macros used in other parts of python_games.cc are deprecated since Sep 2020 (pybind/pybind11#2325). It is recommended to replace them with the equivalent `PYBIND11_OVERRIDE_*` macros.

PiperOrigin-RevId: 518266312
Change-Id: Ica90764a71b8ed7795b2f26ab02a904bf2ad901e

Author: rwgk

open_spiel/python/pybind11/python_games.cc

@@ -369,11 +369,10 @@ std::string PyState::Serialize() const {
 }
 
 int PyState::MeanFieldPopulation() const {
-  // Use a python population() implementation if available.
-  PYBIND11_OVERRIDE_IMPL(int, State, "mean_field_population");
-
-  // Otherwise, default to behavior from the base class.
-  return State::MeanFieldPopulation();
+  // Use a Python implementation if available, fall back to the C++
+  // implementation if not.
+  PYBIND11_OVERRIDE_NAME(int, State, "mean_field_population",
+                              MeanFieldPopulation, /* no arguments */);
 }
 
 }  // namespace open_spiel