_wait_for_msg() on CPython with TLS connection raises ssl.SSLWantReadError on timeout

When connected via TLS, i.e. the socket is wrapped using the `ssl_context` and if there is nothing to be read in the loop, it results in:
```
Traceback (most recent call last):
File "c.py", line 132, in
mqtt_client.loop()
File "/Users/vladimirkotal/Pi/Adafruit_CircuitPython_MiniMQTT/adafruit_minimqtt/adafruit_minimqtt.py", line 955, in loop
rc = self._wait_for_msg(timeout)
File "/Users/vladimirkotal/Pi/Adafruit_CircuitPython_MiniMQTT/adafruit_minimqtt/adafruit_minimqtt.py", line 976, in _wait_for_msg
res = self._sock_exact_recv(1)
File "/Users/vladimirkotal/Pi/Adafruit_CircuitPython_MiniMQTT/adafruit_minimqtt/adafruit_minimqtt.py", line 1077, in _sock_exact_recv
recv_len = self._sock.recv_into(rc, bufsize)
File "/usr/local/Cellar/python@3.9/3.9.13_1/Frameworks/Python.framework/Versions/3.9/lib/python3.9/ssl.py", line 1242, in recv_into
return self.read(nbytes, buffer)
File "/usr/local/Cellar/python@3.9/3.9.13_1/Frameworks/Python.framework/Versions/3.9/lib/python3.9/ssl.py", line 1100, in read
return self._sslobj.read(len, buffer)
ssl.SSLWantReadError: The operation did not complete (read) (_ssl.c:2633)
```

In such case, `_wait_for_msg()` should return `None`, however the ssl exception fires through: https://github.com/adafruit/Adafruit_CircuitPython_MiniMQTT/blob/342b8c99b2c8f10b605cc8311bf5f394f4d18e92/adafruit_minimqtt/adafruit_minimqtt.py#L967-L972

Note that the line numbers do not match because I am using minimqtt code for one of my PRs, however that should not matter.

Trouble is that the `ssl.SSLWantReadError` does not seem to be reachable from the ssl context passed to `MQTT()` init function. The exception class is based on `OSError`, however catching that would be too generic, I think.

Start in adafruit_minimqtt/adafruit_minimqtt.py at _wait_for_msg() and _sock_exact_recv(), following the TLS socket path described in the traceback. Reproduce an idle TLS connection timeout and determine how the SSLWantReadError should be handled so _wait_for_msg() returns None without catching unrelated OSError cases.

Written by the indexing model from the issue text.