The following code causes refleaks:
import zipimport
zi = zipimport.zipimporter.__new__(zipimport.zipimporter)
zi.__init__('bar.zip')
zi.__init__('bar.zip')
zi.__init__('bar.zip\\foo')

This is because zipimport_zipimporter___init___impl() (in Modules/zipimport.c)
doesn't decref (if needed) before assigning to `self->files`, `self->archive`
and `self->prefix`.

I would open a PR to fix this soon.

Should i add a test to test_zipimport?
If yes, could you point out some similar refcount test to help me write this
test?