Package python-pip is required to run globaleaks.

GlobaLeaks attempts to import pip to assert that all dependencies have
been installed; otherwise, it will fail.
Use "install_requires" instead of "requires" in setup.py.

install_requires is used to generate requires.txt file, later read by
dh_python2, and then guess ${Python:Depends}. No idea what that generic
"requires" is useful for.