[python] Cleanup ThreadPool with atexit rather than __del__ (#5094)

* [python] Cleanup ThreadPool with atexit rather than __del__

This removes the `__del__` function from the generated Python client,
and replaces it with a `cleanup` function. When a ThreadPool is created,
the cleanup function is registered with the `atexit` module.

This fixes #5093, where the API client could hang indefinitely at
garbage collection.

* Update petstore examples

* Test to ensure threadpool is cleaned up

* Docs now encourage using the context manager

* Regenerate docs

* Update samples

samples/client/petstore/python-asyncio/README.md

@@ -54,17 +54,19 @@ from pprint import pprint
 
 # Defining host is optional and default to http://petstore.swagger.io:80/v2
 configuration.host = "http://petstore.swagger.io:80/v2"
-# Create an instance of the API class
-api_instance = petstore_api.AnotherFakeApi(petstore_api.ApiClient(configuration))
-body = petstore_api.Client() # Client | client model
-
-try:
-    # To test special tags
-    api_response = api_instance.call_123_test_special_tags(body)
-    pprint(api_response)
-except ApiException as e:
-    print("Exception when calling AnotherFakeApi->call_123_test_special_tags: %s\n" % e)
+# Enter a context with an instance of the API client
+with petstore_api.ApiClient(configuration) as api_client:
+    # Create an instance of the API class
+    api_instance = petstore_api.AnotherFakeApi(api_client)
+    body = petstore_api.Client() # Client | client model
 
+    try:
+        # To test special tags
+        api_response = api_instance.call_123_test_special_tags(body)
+        pprint(api_response)
+    except ApiException as e:
+        print("Exception when calling AnotherFakeApi->call_123_test_special_tags: %s\n" % e)
+    
 ```
 
 ## Documentation for API Endpoints