Changed

• ngclient: default user-agent was updated from "tuf/x.y.z" to "python-tuf/x.y.z" (theupdateframework#2632)
• ngclient: max_root_rotations default value was bumped to 256 to prevent a too small value
  from creating issues in actual deployments were the embedded root is not easily
  updateable (theupdateframework#2675)
• repository: do_snapshot() and do_timestamp() now always create new versions if current version
  is not correctly signed (theupdateframework#2650)
• Various infrastructure and documentation improvements