Temporarily pin pip to any version <20
Temporarily pin pip to any version <20

There was an issue recently introduced with pip 20.0.1 that prevents
some binary wheels from being installed. This will result in failed
setup of angr-dev. Until it gets resolved, simply use a slightly older
version of pip.

More info: https://github.com/pypa/pip/issues/7629

angr/angr-dev,angr/angr-dev