lib: Fix safeRef regression after removing __cmp__ in #4684

[echoix]

After stepping through a lot with PDB, I suggest this to fix the hang described in #4684 (comment) and #4684 (comment)

It isn't a question of missing __le__, __ge__, __ne__ as the replacement of __cmp__ with all rich comparisons. They don't get called. The current (before #4684) __cmp__ method isn't called at all in Python 3 (breakpoint isn't hit, while other do).
In #4684, the __eq__ method gets called by the list.index from the receivers.index(receiver) in here:

grass/python/grass/pydispatch/dispatcher.py

Lines 448 to 478 in 7f1934f

	def _removeOldBackRefs(senderkey, signal, receiver, receivers):
	"""Kill old sendersBack references from receiver
	This guards against multiple registration of the same
	receiver for a given signal and sender leaking memory
	as old back reference records build up.
	Also removes old receiver instance from receivers
	"""
	try:
	index = receivers.index(receiver)
	# need to scan back references here and remove senderkey
	except ValueError:
	return False
	else:
	oldReceiver = receivers[index]
	del receivers[index]
	found = 0
	signals = connections.get(signal)
	if signals is not None:
	for sig, recs in connections.get(signal, {}).items():
	if sig != signal:
	for rec in recs:
	if rec is oldReceiver:
	found = 1
	break
	if not found:
	_killBackref(oldReceiver, senderkey)
	return True
	return False

Looking at the locals() available in debugging at various stack levels, I see no real difference between having the __eq__ method after #4684, or removing it completly (by memory of what I saw multiple dozens of times per code-change though, so I might be wrong). However, without any extra method, it works, on Linux (ubuntu 22.04 container on WSL) and on windows (editing the saferef.py file in an OSGeo4W grass-dev build 432). I cannot test on macOS, but I suppose it is the same.

This PR effectively has the same behavior as before #4684 for python 3, as that method didn't get called.
From what I searched, it really should have only been called, in python 2, if a list was sorted. I didn't find any other ways the methods in saferef could get triggered.

I consider this change safe.

Additionnaly, in my first attempts to debug this, I also tried raising warnings (similar to DeprecationWarning), with PYTHONWARNINGS=always env var on all methods in saferef, and other methos appeared, but never __cmp__, or the others added in #4684, of course, __eq__ was called, as it was the problematic part

[nilason]

Works for me, thanks!