Expose __bytes__ on Key protocols

My first attempt (inheriting from `SupportsBytes`) was fine as far as
mypy was concerned. But this failed with an MRO problem.

I suspect you're not supposed to actually inherit from `SupportsBytes`.
Or maybe `SupportsBytes` is itself a `Protocol` and there was some
inheritance ambiguity there.

Author: David Robertson

signedjson/types.py

@@ -30,6 +30,9 @@ class BaseKey(Protocol):
     version = ""  # type: str
     alg = ""  # type: str
 
+    def __bytes__(self) -> bytes:
+        pass
+
     def encode(self):
         # type: () -> bytes
         pass  # pragma: nocover