Allow "Required: False" on Response headers - Fix spec-first#1261 (sp…

…ec-first#1293)

* Add failing test for optional header

* Required: False headers don't give error if missing

* Use dict.get

* Required keys are when they are not present, or present with "True"

* Update connexion/decorators/response.py

Co-authored-by: Ruwann <ruwan.lambrichts@ml6.eu>

* Update tests for required and optional headers

Swagger2 response headers are optional, and have no 'required'
attribute. OpenAPI3 response headers are optional by default,
but can be specified to be required by setting 'required: true'.

* Make headers required for tests that rely on it

Co-authored-by: Ruwann <ruwan.lambrichts@ml6.eu>

Add readthedocs config with docs requirements (spec-first#1391)

 spec-first#1182 get rid of @asyncio.coroutine (deprecated in Python …

…3.8) (spec-first#1186)

Signed-off-by: Sławomir Zborowski <slawomir.zborowski@nokia.com>

fix nested additionalProperties (spec-first#1138)

Merge pull request spec-first#1109 from spacether/issue_1108_py27_fix

Don't allow connexion installation in py27

use non-signed tag

drop Python 3.5, add 3.8 (spec-first#1056)

* drop Python 3.5, add 3.8

* remove old pypy 2.7 (no longer available on 18.04)