py-OpenSSL: updated to 18.0.0

18.0.0:
Backward-incompatible changes:
- The minimum cryptography version is now 2.2.1.
- Support for Python 2.6 has been dropped.

Changes:
- Added Connection.get_certificate to retrieve the local certificate.
- OpenSSL.SSL.Connection now sets SSL_MODE_AUTO_RETRY by default.
- Added Context.set_tlsext_use_srtp to enable negotiation of SRTP keying material.

(adam)