* Deal with mypy errors w/ type-hinted pynacl 1.5.0

Fixes #11644.

I really don't like that we're monkey patching pynacl SignedKey
instances with alg and version objects. But I'm too scared to make the
changes necessary right now.

(Ideally I would replace `signedjson.types.SingingKey` with a runtime class which
wraps or inherits from `nacl.signing.SigningKey`.) C.f. https://github.com/matrix-org/python-signedjson/issues/16