Fix some issues in cryptography package

[ilevkivskyi]

Note: the verifier() methods are not in docs, but they don't look private and are actually used in some code.

@JelleZijlstra @srittau
I think we should try including the cryptography stubs in the upcoming release, so if you have time please scroll through these stubs maybe you will spot some more issues.

[srittau]

LGTM, no red flags. The only thing I noticed (already in the original contribution): cryptography.x509 is a package in implementation and a module here, but this can be split later, if required.