py-foolscap: update to 0.12.7

Release 0.12.7:
This is a minor bugfix release to help Tahoe-LAFS.

It depends upon a newer version of I2P, which should handle Tahoe storage
servers that listen on I2P sockets (the Tahoe executable makes an outbound
connection to the local I2P daemon, however it then accepts inbound TLS
connections on that same socket, which confuses the TLS negotiation because
both sides appear to be "clients", and TLS requires exactly one "client" and
one "server").

It also fixes a minor Tub shutdown behavior to let unit tests work more
reliably.

(adam)