Starting point for pybind11 bindings

[lamiller0]

Very very early WIP to start building out pybind11 versions of CMake

Bring over a bunch of the constants, but declare them as pybind11 enums.

I'm not sure yet how best to expose some of the stuff PyAlembic will eventually need, and I needed to put PyImathVec.cpp in MODSOURCE as the register symbols were not being found in LIBSOURCE.

[linux-foundation-easycla]

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[cary-ilm]

Thanks for this! The ASWF projects require signed commits, or DCO (Developer Certificate of Origin), which basically means creating commits with the -s option, which adds a line certifying that the change originated with you. You should be able to add this via:

% git commit --amend -s
% git push --force

[cary-ilm]

We started a side email discussion about the feature branch, and @lgritz advised against a feature branch, which makes sense. How about we discard the pybind11 branch and merge this into main, but create an entirely new directory parallel to src/python called src/pybind11 so that we don't intermingle the new pybind11 with the existing boost implementation. Then a single top-level CMake option to add ``src/pybind11` subdirectory. That should make it easy to support development on the new stuff while still maintaining/releasing the current stuff.

I'm open to other suggestions, but I'd prefer to not introduce a pybind11 dependency until we're ready.

[cary-ilm]

Also, we'll need the CI to install pybind11, since as it is now, the build is not finding it so the checks are ignoring the new library. That should presumably be done in the docker container.

[cary-ilm]

CMake is failing for me on ImathTestC:

CMake Error at src/python/PyImathTest/CMakeLists.txt:23 (target_link_libraries):
  Target "PyImathTestC" links to:

    Imath::PyImath_Python3_10

  but the target was not found.  Possible reasons include:

    * There is a typo in the target name.
    * A find_package call is missing for an IMPORTED target.
    * An ALIAS target is missing.

This may be a problem with how I have boost installed on my machine, but just confirming, this works for you?

[lamiller0]

My initial intention of putting it in src/python was because I was hoping to be able to easily use PyImathTest, but looking a bit closer at it I realize now that its not python unit tests. I dont mind moving it into an adjacent pybind11 and making the other changes you requested.