Revert "_ctypes: don't cache library, bug #448858"

This reverts commit 9e37cca4f54260bd8c45a3041fcee00938c71649.
As noted in bug #448858, comment #14, dlclose is not called
automatically, so we may as well cache our library handles.
In order to protect ourselves, we use a fork since commit
7ebb2f54877edb28621c33e380f8777b1b1dc201.