setup.py: enable 'embedsignature' cython directive to allow doctests
setup.py: enable 'embedsignature' cython directive to allow doctests

To use pytest-cython the embedsignature compiler directive must be set
to True:
https://github.com/lgpage/pytest-cython
http://docs.cython.org/en/latest/src/userguide/source_files_and_compilation.html#compiler-directives

fonttools/skia-pathops