v6.0.0

This release is not strictly speaking an API break from 5.1 but it does contain some
major internal changes that users should be aware of when upgrading.

Changed

• ngclient: urllib3 is used as the HTTP library by default instead of requests (#2762,
  #2773, #2789)
  • This removes dependencies on requests, idna, charset-normalizer and certifi
  • The deprecated RequestsFetcher implementation is available but requires selecting
    the fetcher at Updater initialization and explicitly depending on requests
• ngclient: TLS certificate source was changed. Certificates now come from operating
  system certificate store instead of certifi (#2762)
• ngclient: The updater can now initialize from embedded initial root metadata every
  time. Users are recommended to provide the bootstrap argument to Updater (#2767)
• Test infrastructure has improved and should now be more usable externally, e.g. in
  distro test suites (#2749)