dsse: Improve type checking

mypy rightly complains our types do not match (this only happen if you
enable type checks for securesystemslib):
* I think the annotation is actually wrong: Envelope does not know the
  contained type at this point.
* Likely SimpleEnvelope should not be generic: it does not relly know
  what it contains

I decided not to break the API here and just made the type cast
explicit (even though we don't really know that the cast is correct):
this silences mypy but has no other consequences.

Signed-off-by: Jussi Kukkonen <[email protected]>

tuf/api/dsse.py

@@ -81,7 +81,7 @@ def from_bytes(cls, data: bytes) -> SimpleEnvelope[T]:
         except Exception as e:
             raise DeserializationError from e
 
-        return envelope
+        return cast(SimpleEnvelope[T], envelope)
 
     def to_bytes(self) -> bytes:
         """Return envelope as JSON bytes.