add `Connection.use_(certificate|privatekey)`

[mhils]

(depends on cryptography 38)

Follow-up to https://github.com/pyca/cryptography/pull/7210. :) I've added comments to point out the duplication between Context and Connection. Let me know if you want those out, happy to adjust to whatever style you prefer.

[reaperhulk]

cryptography 38 is out now 😄

[reaperhulk]

Conflict here due to the new pinning, but we should get this reviewed ASAP 😄

[mhils]

Conflict fixed! 😃