Drop aiohttp support

[RobbeSneyders]

Closes #1490

[coveralls]

Pull Request Test Coverage Report for Build 2001019325

• 1 of 3 (33.33%) changed or added relevant lines in 1 file are covered.
• 97 unchanged lines in 10 files lost coverage.
• Overall coverage decreased (-3.5%) to 93.264%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
connexion/apis/abstract.py	1	3	33.33%

Files with Coverage Reduction	New Missed Lines	%
connexion/operations/openapi.py	1	95.29%
connexion/cli.py	2	95.4%
connexion/options.py	2	77.42%
connexion/decorators/response.py	5	89.09%
connexion/apis/abstract.py	6	88.83%
connexion/operations/secure.py	6	73.12%
connexion/jsonifier.py	9	70.0%
connexion/security/security_handler_factory.py	10	81.73%
connexion/decorators/decorator.py	11	55.56%
connexion/security/async_security_handler_factory.py	45	19.67%

Totals
Change from base Build 1994636488:	-3.5%
Covered Lines:	2520
Relevant Lines:	2702

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💛 - Coveralls