import_package defines a signal handler violating test isolation

[Vincent Ladeuil]

Long story short:
- import_package defines a signal handler
- interrupting selftest.py outputs a traceback ending in import_package.py
- heart attack: OMG! Real imports triggered in unrelated tests ! Wtf ?

Setting importance to medium as this is only a minor inconvenience for now and has been the case since import_package is imported by selftest.

This may need to be bumped as it will cause problems if we write more tests (not to mention that mass_import also defines the same signal handler ;)