This is a pretty big simplification: the user should first use pip to
install everything in `requirements.txt`, and we no longer try to
cater to cases where Cython is missing, etc.
simplejson was there for python <= 2.6 compatibility.
It's not needed now, and installing it with pip can lead to segfaults
because pip is dumb.
(see e.g. https://github.com/simplejson/simplejson/issues/114)
