src/untangle/untangle.py: use cleaner way of disabling verification.

In the past, the internal _create_unverified_context() method of the
ssl module was used to create a context that doesn't verify
certificates. However, (now?) there is a cleaner way: by setting the
context's "check_hostname" member to False and its "verify_mode"
member to ssl.CERT_NONE. Which is exactly what this commit does.