Fix for IPRoute objects that can still be used after close in Python 2

[etene]

Hello,

I noticed IPRoute contexts could still be used after being closed in Python 2. I assume I'm not the first one to stumble upon this, but it has caused inconsistent behavior between Python versions in our codebase so I figured it warranted a fix.

The gist of the issue is that NetlinkSocket stores references to the underlying socket methods, which can change (and do). So instead of storing references I used __getattr__ to always return the "current" methods.

The reference-based approach caused IPRoute sockets to be in an inconsistent state when closed:

• the socket methods are replaced by dummies at closing time
• the stored reference still points to the old method
• the underlying os-level socket is not necessarily closed
  Thus data still could be sent over the socket even if it was considered closed.

Only Python 2 is affected, because in Python 3 socket methods point to the "real thing" and are not replaced by dummies when closing.

I guess a __getattr__ call could have a performance overhead compared to the old method, so tell me if this is a problem and I'll think of another way.

Cheers,
Étienne

[svinota]

Thanks! Testing

[svinota]

Thanks!