Import the setools classes needed for Python bindings from specific
setools modules in order to reduce the dependency footprint
of the Python bindings. Importing the top-level module causes all
setools modules to be loaded which includes the modules that require
networkx.
SELinux packages belong to the group of core system packages on Gentoo
Linux. It is desirable to keep the system set as small as possible,
and the dependency between setools and networkx seems to be the easiest
link to break without major loss of functionality.
Signed-off-by: Michał Górny <mgorny@gentoo.org>