Add async-generator to tests_require (#2947)
Add async-generator to tests_require (#2947)

This test dependency was introduced in 374f173448d2a7ffc01b6c4e0c8ea65c08f49b78.
arthurdarcet/aiohttp,arthurdarcet/aiohttp,KeepSafe/aiohttp,KeepSafe/aiohttp,rutsky/aiohttp,rutsky/aiohttp,rutsky/aiohttp,KeepSafe/aiohttp,arthurdarcet/aiohttp