Deal with mypy errors w/ type-hinted pynacl 1.5.0 (#11714)

* 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

changelog.d/11714.misc

@@ -0,0 +1 @@
+Fix a typechecker problem related to our (ab)use of `nacl.signing.SigningKey`s.

tests/crypto/test_event_signing.py

@@ -14,6 +14,7 @@
 
 
 import nacl.signing
+import signedjson.types
 from unpaddedbase64 import decode_base64
 
 from synapse.api.room_versions import RoomVersions
@@ -35,7 +36,12 @@ HOSTNAME = "domain"
 
 class EventSigningTestCase(unittest.TestCase):
     def setUp(self):
-        self.signing_key = nacl.signing.SigningKey(SIGNING_KEY_SEED)
+        # NB: `signedjson` expects `nacl.signing.SigningKey` instances which have been
+        # monkeypatched to include new `alg` and `version` attributes. This is captured
+        # by the `signedjson.types.SigningKey` protocol.
+        self.signing_key: signedjson.types.SigningKey = nacl.signing.SigningKey(
+            SIGNING_KEY_SEED
+        )
         self.signing_key.alg = KEY_ALG
         self.signing_key.version = KEY_VER