paho-mqtt-2.0.0 client issue

# Bug Description

When running the script with paho-mqtt 2.0.0 and initializing the Client(), an AttributeError occurs: 'Client' object has no attribute '_sock'. However, when changing to paho-mqtt 1.6.1, everything works fine.

# Environment

* Python version: 3.8
* Library version: 2.0.0
* Operating system (including version): Ubuntu 20.04.6
* MQTT server (name, version, configuration, hosting details): 2.0.18

# Logs
Exception ignored in:
Traceback (most recent call last):
File "/usr/local/lib/python3.8/site-packages/paho/mqtt/client.py", line 874, in __del__
self._reset_sockets()
File "/usr/local/lib/python3.8/site-packages/paho/mqtt/client.py", line 1133, in _reset_sockets
self._sock_close()
File "/usr/local/lib/python3.8/site-packages/paho/mqtt/client.py", line 1119, in _sock_close
if not self._sock:
AttributeError: 'Client' object has no attribute '_sock'