Apig-wsgi

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* Support Python 3.9.
* Fix query string parameter encoding so that symbols are correctly re-encoded
for WSGI, for API Gateway format version 1
(`Issue 186 <https://github.com/adamchainz/apig-wsgi/pull/186>`__).

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* Move license from ISC to MIT License.
* Always send ``isBase64Encoded`` in responses, as per the AWS documentation.
* Support `format version
2 <https://docs.aws.amazon.com/apigateway/latest/developerguide/http-api-develop-integrations-lambda.html>`,
which was introduced by API Gateway for “HTTP API's”
(`Issue 124 <https://github.com/adamchainz/apig-wsgi/pull/124>`__)..
* ``binary_support`` now defaults to ``None``, which means that it will
automatically enable binary support for format version 2 events.

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* Use multi-value headers in the response when given in the request.
(`PR 157 <https://github.com/adamchainz/apig-wsgi/pull/157>`__).

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* Add defaults for ``SERVER_HOST``, ``SERVER_PORT`` and ``wsgi.url_scheme``.
This enables responding to `ELB health check events
<https://docs.aws.amazon.com/elasticloadbalancing/latest/application/lambda-functions.html#enable-health-checks-lambda>`__,
which don't contain the relevant headers
(`Issue 155 <https://github.com/adamchainz/apig-wsgi/pull/155>`__).

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* Pass Lambda Context object as a WSGI environ variable
(`PR 122 <https://github.com/adamchainz/apig-wsgi/pull/122>`__).

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* Restore setting ``HTTP_CONTENT_TYPE`` and ``HTTP_HOST`` in the environ,
accidentally broken in 2.5.0 with multi-value header feature
(`Issue 117 <https://github.com/adamchainz/apig-wsgi/issues/117>`__).