Gidgethub

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- :meth:`gidgethub.abc.GitHubAPI.post` now accepts ``content_type`` parameter.
If supplied, the ``content_type`` value will be used in the request headers,
and the raw form of the data will be passed to the request. If not supplied,
by default the data will be parsed as JSON, and the "application/json" content
type will be used. (`Issue 115 <https://github.com/brettcannon/gidgethub/issues/115>`_).

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- Fix :meth:`gidgethub.abc.GitHubAPI.graphql` to accept response content types lacking
spaces; affects GitHub Enterprise
(`Issue 122 <https://github.com/brettcannon/gidgethub/pull/122>`_)

- Specify PyJWT[crypto] as a dependency, and dropped the explicit dependency to
``cryptography`` (`Issue 116 <https://github.com/brettcannon/gidgethub/issues/116>`_).

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- Introduce :mod:`gidgethub.apps`.

- Introduce :meth:`gidgethub.apps.get_installation_access_token`
for obtaining an installation access token that can be used to authenticate as
a GitHub App installation. (`Issue 71 <https://github.com/brettcannon/gidgethub/issues/71>`_).

- Introduce :meth:`gidgethub.apps.get_jwt` for constructing the JSON Web Token
that can be used to access endpoints that require it.

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- Add :meth:`gidgethub.abc.GitHubAPI.graphql` and related exceptions.
- Add :exc:`gidgethub.BadRequestUnknownError` when something other than JSON is
returned for a 422 response.
- Remove `gidgethub.treq`; tests were not passing and a request for help on
Twitter came back with no response (happy to add back if someone steps forward
to help out).
- Remove `gidgethub.test` from the distribution.
- Introduce :mod:`gidgethub.actions`.
- Add :exc:`gidgethub.ValidationError` for when the HTTP response is a 422 but not
field-related. (`83 <https://github.com/brettcannon/gidgethub/pull/83>`_;
thanks `John Hossler <https://github.com/jmhossler>`_)
- Allow GitHub Enterprise users to specify a base URL, by passing in **base_url**
to the :meth:`gidgethub.abc.GitHubAPI` constructor.

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- Adapt to the new ``httpx`` API to support
`versions >= 0.11.0 <https://github.com/encode/httpx/blob/master/CHANGELOG.md>`_
(thanks `Dave Hirschfeld <https://github.com/dhirschfeld>`_)

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- Fix mypy warnings about the :class:`typing.Dict` and :class:`typing.Mapping`
generic types lacking type parameters.
- Add :mod:`gidgethub.httpx` backend.
- Add a ``py.typed`` file to mark the project as fully typed.