addset: drop caching through generatorset
The utility of this cache is debatable (no visible benchmark impact) and using
generatorset for such purpose makes the code complicated.
We drop it for now. Someone can reintroduce a smart version of it in the future
if it is detected to be relevant.
try:
import coverage
getattr(coverage, 'process_startup', lambda: None)()
except ImportError:
pass