5.1.0

• Use X-Hub-Signature-256 header for webhook validation when available.
  (PR #160).

• The documentation is now built using Sphinx v>= 4.0.0.
  (Issue #143)

• gidgethub.abc.GitHubAPI.getiter now accepts iterable_key parameter
  in order to support the Checks API.
  (Issue #164)

• Accept HTTP 202 ACCEPTED as successful.
  (PR #174)