python install, a more robust pybind11 include
python install, a more robust pybind11 include

Summary: In some environments, `python setup.py install` fails to install pybind11. The solution is given by `pybind11`'s repository which consists on calling `pip install pybind11` via subprocess. The issue and the solution were reported here https://github.com/facebookresearch/fastText/issues/512

Reviewed By: EdouardGrave

Differential Revision: D13167381

fbshipit-source-id: 4ee7835a07e503d00728857242e085bc7de53c14

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