Nuke an unnecessary duplication of a FedmsgMessage every time we sign()

Signed-off-by: Ricky Elrod <ricky@elrod.me>
Bump version to 3 because I am dumb and 9361766 should have been in 2.0.0 and breaks the API.

Signed-off-by: Ricky Elrod <ricky@elrod.me>