Mark key types as SupportsBytes

[DMRobertson]

pynacl's SigningKey and VerifyKey both expose a __bytes__ method, so
this is correct. It's just that the information gets lost when we
replace these with our Protocol types.

While I'm at it, add a drive-by-comment.

Blocks matrix-org/synapse#12326.

[DMRobertson]

We don't have CI set up for this repo. I ran mypy manually and it didn't find anything alarming:

 $ mypy 
signedjson/__init__.py:18: error: Incompatible import of "PackageNotFoundError" (imported name has type "Type[importlib_metadata.PackageNotFoundError]", local name has type "Type[importlib.metadata.PackageNotFoundError]")  [misc]
signedjson/__init__.py:18: note: Error code "misc" not covered by "type: ignore" comment
signedjson/key.py:21: error: Cannot find implementation or library stub for module named "unpaddedbase64"  [import]
signedjson/key.py:21: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
signedjson/sign.py:23: error: Cannot find implementation or library stub for module named "unpaddedbase64"  [import]
tests/test_sign.py:19: error: Cannot find implementation or library stub for module named "unpaddedbase64"  [import]
tests/test_known_key.py:20: error: Cannot find implementation or library stub for module named "unpaddedbase64"  [import]
Found 5 errors in 5 files (checked 8 source files)