Fix TypeError in signals module on weak object finalize by GC

[rvtpro]

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master or python-dual-support branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment
• I've included docstrings and/or documentation and/or examples for my code (if this is a new feature)

Description:

Fixed a bug in signal module with weakref callback and added some tests for signal module.
How to reproduce error:

import urwid
def handler(weak_debug, widget, newtext):
    weak_debug.set_text("Edit widget changed to %s" % newtext)

debug = urwid.Text('')
edit = urwid.Edit('')
key = urwid.connect_signal(edit, 'change', handler, weak_args=[debug])
del debug

throws

Exception ignored in: <function Signals.connect.<locals>.weakref_callback at 0x7f97bdf44f70>
Traceback (most recent call last):
  File ".../urwid/signals.py", line 178, in weakref_callback
    del getattr(o, self._signal_attr, {})[name][key]
TypeError: list indices must be integers or slices, not Key

[neumond]

👍
I've applied similar monkeypatch in my project. It would be nice to see this merged.