Add pyproject.toml to comply with PEP-518 (#1292)

Summary:
Adds pyproject.toml to comply with PEP-518, which fixes the building of the library by poetry - See https://github.com/python-poetry/poetry/issues/6113 . This is a copy of https://github.com/facebookresearch/fastText/pull/1270 , but I have signed the CLA.

Pull Request resolved: https://github.com/facebookresearch/fastText/pull/1292

Differential Revision: D51601444

Pulled By: alexkosau

fbshipit-source-id: 357d702281ca3519c3640483eba04d124d0744b4

pyproject.toml

@@ -0,0 +1,2 @@
+[build-system]
+requires = ["setuptools", "wheel", "pybind11"]