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 PR also allows the client to be used as a context manager, which
will automatically clean up after itself rather than having to wait til
process exit.

This fixes issue #1037, where the API client could hang indefinitely at
garbage collection.

kubernetes/client/api_client.py

@@ -10,6 +10,7 @@
 
 from __future__ import absolute_import
 
+import atexit
 import datetime
 import json
 import mimetypes
@@ -77,11 +78,19 @@ class ApiClient(object):
         # Set default User-Agent.
         self.user_agent = 'OpenAPI-Generator/11.0.0-snapshot/python'
 
-    def __del__(self):
+    def __enter__(self):
+        return self
+
+    def __exit__(self, exc_type, exc_value, traceback):
+        self.close()
+
+    def close(self):
         if self._pool:
             self._pool.close()
             self._pool.join()
             self._pool = None
+            if hasattr(atexit, 'unregister'):
+                atexit.unregister(self.close)
 
     @property
     def pool(self):
@@ -89,6 +98,7 @@ class ApiClient(object):
          avoids instantiating unused threadpool for blocking clients.
         """
         if self._pool is None:
+            atexit.register(self.close)
             self._pool = ThreadPool(self.pool_threads)
         return self._pool