publication of the sdist file on pypi does not allow on-the-fly build

[johannct]

The problem lies in the fact that the sdist does not include the populated extern/pybind11 directory. The root cause seems to be that sdist by default is created by gathering default dirs, like the src directory, and does not grab the extern directory. One quick fix is to write a MANIFEST.in file, with just the following inside :

recursive-include extern *