python: remove non-async code path from the aiohttp generator (#16601)

* 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: #15824
Closes: #5539
Related: #763
Related: #3696

* Fix empty line

* remove more

samples/openapi3/client/petstore/python-aiohttp/tests/test_api_client.py

@@ -15,13 +15,8 @@ class TestApiClient(unittest.TestCase):
     async def test_context_manager_closes_client(self):
 
         async with petstore_api.ApiClient() as client:
-            # thread pool
-            self.assertIsNotNone(client.pool)
-            pool_ref = weakref.ref(client._pool)
-            self.assertIsNotNone(pool_ref())
             # pool_manager
             self.assertFalse(client.rest_client.pool_manager.closed)
             rest_pool_ref = client.rest_client.pool_manager
 
-        self.assertIsNone(pool_ref())
         self.assertTrue(rest_pool_ref.closed)

samples/openapi3/client/petstore/python-aiohttp/tests/util.py

@@ -11,7 +11,8 @@ def id_gen(bits=32):
 
 def async_test(f):
     def wrapper(*args, **kwargs):
-        coro = asyncio.coroutine(f)
+        # coro = asyncio.coroutine(f)
+        coro = f
         future = coro(*args, **kwargs)
         loop = asyncio.get_event_loop()
         loop.run_until_complete(future)