Allow providing custom ssl context

[rhegner]

Connecting to my router I got a SSLV3_ALERT_HANDSHAKE_FAILURE. The root cause seems to be an incompatible cipher being used. See

• SSLV3_ALERT_HANDSHAKE_FAILURE (despite verify_ssl=False) #404
• https://stackoverflow.com/a/73254780/487356

This PR allows to create a SagemcomClient with a custom SSL context like this:

sslcontext = ssl._create_unverified_context()
sslcontext.set_ciphers("AES256-GCM-SHA384")
SagemcomClient(HOST, USERNAME, PASSWORD, ENCRYPTION_METHOD, verify_ssl=None, ssl_context=sslcontext)

Note that from a TCPConnector perspective, verify_ssl and ssl_context are mutually exclusive parameters. That's why I had to change the handling of verify_ssl slightly, to allow passing in None.

[rhegner]

Turns out that this is not needed, see /docs/ssl-error.md