X509Backend support in OpenSSL backend

This PR adds a dependency on `enum34`. I am expecting some improvements to be needed in the docs along with potentially some comments required to explain the purpose of some of the tests.

TODO:
- [x] Resolve issue with EC public keys where curve name is not set. (resolved by raising error for now)

Depends on #1498.
Shit I just realized this, but should this checking for a thing with an unnamed curve be in `_evp_pkey_to_public_key`? It seems like it could occur with regular key loading as well?