Apig-wsgi

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* Change ``statusCode`` returned to API Gateway / ALB to an integer. It seems
API Gateway always supported both strings and integers, whilst ALB only
supports integers.
* Add ``statusDescription`` in return value. API Gateway doesn't seem to use
this whilst the `ALB documentation <https://docs.aws.amazon.com/elasticloadbalancing/latest/application/lambda-functions.html>`_
mentions it as supported.

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* Drop Python 2 support, only Python 3.4+ is supported now.

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* Temporarily restore Python 2 support. This is in order to fix a packaging
metadata issue that 2.0.0 was marked as supporting Python 2, so a new release
is needed with a higher version number for ``python -m pip install apig-wsgi`` to
resolve properly on Python 2. Version 2.0.2+ of ``apig-wsgi`` will not
support Python 2.

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* Drop Python 2 support, only Python 3.4+ is supported now.
* If ``exc_info`` is passed in, re-raise the exception (previously it would be
ignored and crash in a different way). This isn't the nicest experience,
however the behaviour is copied from ``wsgiref``\'s simple server, and most
WSGI applications implement their own exception conversion to a "500 Internal
Server Error" page already.
* Noted that the EC2 ALB to Lambda integration is also supported as it uses the
same event format as API Gateway.

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* Work with base64 encoded ``body`` values in requests from API Gateway.

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* Fix crash using binary support for responses missing a ``Content-Type``
header.