Set socket timeout to 0 by default.

As long as the timeout is >= 0, Python's _ssl module will still use
non-blocking I/O but not poll() but select(). poll(&fd, 1, TIMEOUT)
seems to block _at least_ TIMEOUT time regardless of any errors.

Currently, with SUSE's internal IPv6 layout, this meant a hang of a
second per binary to fetch during "osc build".