Pull in type hints for pynacl

[DMRobertson]

In my spare time (i.e. not paid Element time) I've been working to add type annotations to pynacl, see pyca/pynacl#660. This is almost done, and the maintainer is keen to include them in a new minor release, "in the next few days/weeks".

Once there's a release, we should remove pynacl from the list of imports we ignore in Sydent's mypy config. Then we should fixup any complaints. The same also holds true for signedjson and synapse.

From a quick glance, there seem to be two complaints:

• we're adding version and alg attributes to pynacl SigningKey instances (both in Sydent and signedjson)
• we're creating a SigningKey with a str rather than a bytes (missed in 2->3 conversion?)

[H-Shay]

Looks like pynacl 1.5.0 is out, so we can go ahead with this.

[DMRobertson]

Many thanks to Olivier for doing this in #484.