aiohttp: Unclosed client session

[chlowell]

Async credentials don't close aiohttp sessions. So a program like this:

cred = ClientSecretCredential(...)
await cred.get_token('some scope')

Outputs this on exit:

[c:\python37\Lib\asyncio\base_events.py] Unclosed client session
client_session: <aiohttp.client.ClientSession object at 0x0000016A8BDB6A58>
[c:\python37\Lib\asyncio\base_events.py] Unclosed connector
connections: ['[(<aiohttp.client_proto.ResponseHandler object at 0x0000016A8BF3A1C8>, 1617014.562)]']
connector: <aiohttp.connector.TCPConnector object at 0x0000016A8BDB6A90>

Related: #7228