[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/tests/test_api_client.py

@@ -11,6 +11,8 @@ $ nosetests -v
 
 import os
 import time
+import atexit
+import weakref
 import unittest
 from dateutil.parser import parse
 
@@ -169,3 +171,17 @@ class ApiClientTests(unittest.TestCase):
         data = [pet]
         result = self.api_client.sanitize_for_serialization(data)
         self.assertEqual(result, list_of_pet_dict)
+
+    def test_context_manager_closes_threadpool(self):
+        with petstore_api.ApiClient() as client:
+            self.assertIsNotNone(client.pool)
+            pool_ref = weakref.ref(client._pool)
+            self.assertIsNotNone(pool_ref())
+        self.assertIsNone(pool_ref())
+
+    def test_atexit_closes_threadpool(self):
+        client = petstore_api.ApiClient()
+        self.assertIsNotNone(client.pool)
+        self.assertIsNotNone(client._pool)
+        atexit._run_exitfuncs()
+        self.assertIsNone(client._pool)