atexit handler leaks connection instances

[douglas-raillard-arm]

While reviewing unrelated code, I realized we leak the connection objects when doing:

devlib/devlib/utils/ssh.py

Line 373 in 228baeb

atexit.register(self.close)

            atexit.register(self.close)

This should be fixable with a WeakMethod, and the callback used to call atexit.unregister()