v0.16.0

Added

• Begin to document architectural and project-wide decisions as Architectural
  Decision Records (ADRs) in docs/adr (#1182, #1203)
• Add Python 3.9 to the CI test matrix (#1200)
• Implement a class for Root metadata in the simple TUF role metadata model in
  tuf.api (#1193)

Changed

• Bump dependencies: cryptography (#1189, #1190), requests (#1210),
  urllib (#1212), cffi (#1222), certifi (#1201), securesystemslib (#1191)
• Simplify the test runner (aggregate_tests) and stop executing unit test
  modules in a random order (#1187)
• Speed up indefinite freeze tests by removing sleep() calls (#1194)
• Adapt to securesystemslib changes in key generation interfaces (#1191)
• Migrate from travis-ci.org to travis-ci.com (#1208)
• Make metadata signatures ordered by keyid, to ensure deterministic signature
  ordering in metadata files (#1217)
• Improve test reliability by using thread-safe Queues, rather than files,
  for process communication (#1198)
• Avoid reading an entire target file into memory when generating target file
  hashes in tuf.client.updater (#1219)
• Remove use of an empty list ([]) as the default argument in a test
  function (#1216)
• Simplified updater logic for downloading and verifying target files (#1202)

Fixed

• Fix threshold computation in _verify_root_self_signed() such that
  signatures by the same root key count only once towards the threshold (#1218)