Remove dependency on flaky

[seifertm]

Flaky is currently a dependency in the testing extra of pytest-asyncio. There are dedicated tests to test the compatibility between pytest-asyncio and flaky.

Pytest-asyncio should be compatible with other plugins, but it should not be responsible to test that compatibility. If every pytest plugin wrote tests against other plugins, we ended up in dependency hell and a lot of work would be done multiple times.

There's a dedicated section in pytest for testing plugin integration. This is the right place to do it.

Moreover, the flaky project seems to be abandoned. box/flaky#192

The goal of this issue is to remove flaky from the the project dependencies.