[Python] Generated code is out of contract - application hangs indefinitely

The generated code snippet from - https://github.com/swagger-api/swagger-codegen/blob/master/modules/swagger-codegen/src/main/resources/python/api_client.mustache#L73 is not allowed:

    def __del__(self):
        if self._pool is not None:
            self._pool.close()
            self._pool.join()

The very simple program of creating a global client, and then just exiting will hang indefinitely on Python3.8. Initially raised as CPython bug under https://bugs.python.org/issue39360, but @pablogsal (core CPython developer) investigated and concluded that this way of cleaning up pools is not supported. Instead one should either allow the user to close or if the user did not do the generated code should do by registering https://docs.python.org/3.8/library/atexit.html callbacks.

The code in question seemed to be added within https://github.com/swagger-api/swagger-codegen/pull/6396 by @tuxbotix with approvers being @scottrw93 @taxpon @frol @mbohlool, so tagging them for context.

Start with modules/swagger-codegen/src/main/resources/python/api_client.mustache around line 73 and reproduce the global-client exit case on Python 3.8. Review the cleanup behavior described in the issue and ensure generated clients do not hang during interpreter shutdown, then verify the simple program exits normally.