Don't import concrete security factories in init

[Ruwann]

The concrete SecurityHandlerFactory implementations require the corresponding libraries to be installed.
When importing them in security.__init__.py, this can raise an ImportError.

For example, when running the helloworld example which uses FlaskApp and shouldn't require aiohttp:

$ python hello.py 
Traceback (most recent call last):
  File "hello.py", line 10, in <module>
    app = connexion.FlaskApp(__name__, port=9090, specification_dir='swagger/')
  File "/home/ruwan/projects/connexion/connexion/__init__.py", line 22, in _required_lib
    raise exc
  File "/home/ruwan/projects/connexion/connexion/__init__.py", line 30, in <module>
    from .apis.flask_api import FlaskApi, context  # NOQA
  File "/home/ruwan/projects/connexion/connexion/apis/flask_api.py", line 13, in <module>
    from connexion.security import FlaskSecurityHandlerFactory
  File "/home/ruwan/projects/connexion/connexion/security/__init__.py", line 8, in <module>
    from .aiohttp_security_handler_factory import AioHttpSecurityHandlerFactory  # NOQA
  File "/home/ruwan/projects/connexion/connexion/security/aiohttp_security_handler_factory.py", line 3, in <module>
    import aiohttp
ModuleNotFoundError: No module named 'aiohttp'

This PR addresses this by not importing the concrete classes in security.__init__.py. Another option is to do something similar to connexion.__init__.py, but I opted for this approach because the SecurityHandlerFactory isn't exposed directly to the end user, but happy to change it so this also uses that approach.

[coveralls]

Coverage increased (+0.004%) to 97.081% when pulling b69d0d6 on Ruwann:bugfix/aiohttp-security into 1012721 on zalando:main.

[RobbeSneyders]

Good catch! Not a big preference for one solution, but I do like the cleaner imports by importing them in __init__.

I think we should update our tests to prevent this from happening again. We can group tests by marking them and run separate test suites for aiohttp & Flask with only the necessary imports installed.

[Ruwann]

Thanks! I have updated the PR to use the approach of catching the ImportErrors in security.__init__.py.

I have also opened issue #1389 to discuss how to best set up the testing suites for optional dependencies, as there are different ways of doing this.

[RobbeSneyders]

Ok sounds good!
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