annotation issues

[jku]

If we enable securesystemslib type checks (which we can't really yet do because secure-systems-lab/securesystemslib#944), it shows two issues in python-tuf:

tuf/api/serialization/json.py:101: error: Item "None" of "str | None" has no attribute "encode"  [union-attr]

This looks trivial to fix: encode_canonical() can return None but only if it's given the output_function argument. We can just do assert canonical_str is not None to silence this

tuf/api/dsse.py:84: error: Incompatible return value type (got "Envelope", expected "SimpleEnvelope[Root]")  [return-value]
tuf/api/dsse.py:84: error: Incompatible return value type (got "Envelope", expected "SimpleEnvelope[Timestamp]")  [return-value]
tuf/api/dsse.py:84: error: Incompatible return value type (got "Envelope", expected "SimpleEnvelope[Snapshot]")  [return-value]
tuf/api/dsse.py:84: error: Incompatible return value type (got "Envelope", expected "SimpleEnvelope[Targets]")  [return-value]

not entirely sure how to fix this one: maybe just typing.cast() call?

[jku]

I think SimpleEnvelope should not even have the contained type (like Root in SimpleEnvelope[Root]): SimpleEnvelope does not really know what it contains, unlike Metadata.

That said, maybe the fix here is still just a cast