This changeset allows for cross-compilation of Python packages. Packages
built with buildPythonPackage are not allowed to refer to the build
machine. Executables that have shebangs will refer to the host.
This reverts commit 7c9ca654801d472563ddac4bf488ebdabc2bb491. The
cypari2 update is not ready. The package is pretty much only used for
sage and the update causes problems:
https://trac.sagemath.org/ticket/26442#comment:33
I've already reverted this once before.