pybind11 fork spewing lots of warnings

[alecjacobson]

The build output is dominated by the same warning:

  In file included from /Users/runner/work/libigl-python-bindings/libigl-python-bindings/build/temp.macosx-11.0-arm64-cpython-38/dihedral_angles.out.cpp:2:
  In file included from /Users/runner/work/libigl-python-bindings/libigl-python-bindings/external/numpyeigen/cmake/../src/npe.h:8:
  /Users/runner/work/libigl-python-bindings/libigl-python-bindings/external/numpyeigen/external/pybind11/include/pybind11/pybind11.h:1078:9: warning: expression result unused [-Wunused-value]
          PYBIND11_EXPAND_SIDE_EFFECTS(add_base<options>(record));
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  /Users/runner/work/libigl-python-bindings/libigl-python-bindings/external/numpyeigen/external/pybind11/include/pybind11/detail/common.h:651:47: note: expanded from macro 'PYBIND11_EXPAND_SIDE_EFFECTS'
  #define PYBIND11_EXPAND_SIDE_EFFECTS(PATTERN) pybind11::detail::expand_side_effects{ ((PATTERN), void(), false)..., false }
                                                ^                                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  /Users/runner/work/libigl-python-bindings/libigl-python-bindings/external/numpyeigen/external/pybind11/include/pybind11/iostream.h:194:12: note: in instantiation of function template specialization 'pybind11::class_<pybind11::detail::OstreamRedirect>::class_<pybind11::module_local>' requested here
      return class_<detail::OstreamRedirect>(m, name.c_str(), module_local())
             ^
  1 warning generated.

I believe this comes from numpyeigen > fwilliams/pybind11 new_numpy_hacks_stable

I'm not sure if updating pybind11 would fix this. It's a little tricky because we're on the new_numpy_hacks_stable branch via numpyeigen. We're a bit behind on numpyeigen, but that repo seems to be currently failing it's build CI and doesn't have releases.

Not sure what the best move is. Maybe playing with these branches locally to see if this warning can go away.