simple_example.py requires updating to correct ssl connection issue

### Problem
Current code throws an error on line 82: 
```
# enable TLS for secure connection
client.tls_set(tls_version=mqtt.client.ssl.PROTOCOL_TLS)
```

### Solution
Change imports:
- remove `from paho import mqtt`
- add `import ssl`

Change line 82:
```
# enable TLS for secure connection
client.tls_set(tls_version=ssl.PROTOCOL_TLS_CLIENT)
```
