* python: remove non-async code path from the aiohttp generator
This removes all the non-async code from the aiohttp generator:
* all the methods that should be asynchronous are marked as `async`
* the `async_req` parameter is gone; calls are directly awaitables now
* the async calls into a thread pool are gone and the thread pool is
gone too (now useless)
Closes: #15824Closes: #5539
Related: #763
Related: #3696
* Fix empty line
* remove more
