[python-client] Thread pool fix (#6396)

* [python-client] Added close() and join() for threadpool at ApiClient destructor.

* Updated stuff for CI

samples/client/petstore/python/docs/StoreApi.md

@@ -94,7 +94,7 @@ This endpoint does not need any parameter.
 
 ### Return type
 
-[**dict(str, int)**](dict.md)
+**dict(str, int)**
 
 ### Authorization
 

samples/client/petstore/python/petstore_api/api_client.py

@@ -72,6 +72,10 @@ class ApiClient(object):
         self.cookie = cookie
         # Set default User-Agent.
         self.user_agent = 'Swagger-Codegen/1.0.0/python'
+    
+    def __del__(self):
+        self.pool.close()
+        self.pool.join()
 
     @property
     def user_agent(self):