remove instead of fix stale zipimport._zip_directory_cache data on PyPy

PyPy's zipimport._zip_directory_cache implementation does not support direct
item assignment, thus breaking our attempts at silently updating the cached zip
archive directory information behind the scene. As a workaround, when using
PyPy, we fall back to simply clearing the stale cached information.

--HG--
extra : amend_source : 991e30244754d8fac042da56ac4cf0ad1a0f50d5