Right now when you try to build terra >=0.8.0 and you don't have Cython
installed it will fail. This is because we rely on cython being present
to build the stochastic swap code. While we have a setuptools
setup_requires defining this dependency it doesn't work because the
dependency on cython is needed before setuptools can resolve that for
us. PEP518 provides a solution for this problem by adding the concept of
a pyproject.toml file which can be used to outline build requirements
which are build time depdencies needed before the setup.py is run. With
this file present when you run:
pip install git+https://github.com/Qiskit/qiskit-terra
(or running on a local checkout)
and cython isn't installed pip will acquire it and use it for building
the sdist.
Matthew Treinish