Gidgethub

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- :meth:`gidgethub.abc.GitHubAPI.getiter` now works with
`GitHub's search API <https://docs.github.com/en/rest/search>`_
(thanks `Pablo Galindo <https://github.com/pablogsal>`_).

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- :meth:`gidgethub.sansio.RateLimit.from_http` returns ``None`` if ratelimit is
not found in the headers.
- Allow authenticating as a GitHub App by using JSON web token.
:func:`gidgethub.sansio.create_headers` now accepts
``jwt`` argument. ``gidgethub.abc.GitHubAPI._make_request``,
:meth:`gidgethub.abc.GitHubAPI.getitem`, :meth:`gidgethub.abc.GitHubAPI.getiter`,
:meth:`gidgethub.abc.GitHubAPI.post`, :meth:`gidgethub.abc.GitHubAPI.patch`,
:meth:`gidgethub.abc.GitHubAPI.put`, and :meth:`gidgethub.abc.GitHubAPI.delete` now
accept ``jwt`` and ``oauth_token`` arguments.

- gidgethub is now packaged using `flit <https://flit.readthedocs.io/en/latest/>`_.

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- Tighten type hints for parameters that have a default of ``None``
but were not typed as :data:`typing.Optional`.

- Tweak code to not change semantics but reach 100% coverage.

- Provide a human-readable string representation of
:class:`gidgethub.sansio.RateLimit`.

- Use the ``message`` data as the error message
if the ``errors`` object was not returned.

- Add the *data* keyword argument to :meth:`gidgethub.abc.GitHubAPI.delete`.

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- Tighten up protections against caching ineligible responses.

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- Expand :meth:`gidgethub.routing.Router.dispatch`.

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- Add support for ``application/x-www-form-urlencoded`` webhook event payloads.
(This also allows for API calls to return this content type, although GitHub
currently does not do that.)

- Introduce :mod:`gidgethub.routing` to help route webhook events to registered
asynchronous callbacks.

- Add type hints.

- Add a *cache* argument to :class:`gidgethub.abc.GitHubAPI`.