Make Transport.connect a classmethod

[orsinium]

We have a number of places where we expect methods to be called in a specific order. It mostly concerns connect methods that are expected to be called as soon as the class is instantiated. You can find places like this by looking for assert self._something statements at the beginning of methods.

One such place is Transport which doesn't work if Transport.connect is not called first. The PR converts Transport.connect into a classmethod. So, now it's a constructor that returns an instantiated and connected Transport. It guarantees that any Transport instance is always connected. Well, or disconnected, but at least we can guarantee that TcpTransport._io_reader is never None.