Explicitly support Flask async routes

[RobbeSneyders]

Fixes #1529

Flask async routes were already supported due to the async logic used for aiohttp. This PR just makes it explicit by:

• Installing Flask with the async extra.
  The additional asgiref dependency it adds is already a subdependency of connexion though, so in practice this has no effect.
• Adding a test checking if async routes work

[coveralls]

Pull Request Test Coverage Report for Build 3085233174

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.5%) to 95.219%

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Totals
Change from base Build 3078058780:	0.5%
Covered Lines:	2868
Relevant Lines:	3012

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💛 - Coveralls