Testing `ca_nothing` for boolean needs to be deprecated

[Araneidae]

As discussed in DiamondLightSource/aioca#45 it would appear that allowing a ca_nothing value to return False to a boolean test triggers unexpected behaviour: it turns out that Exception subclasses are expected by core Python libraries to return True when tested with __bool__.

The obvious fix is to delete the implementation of ca_nothing.__bool__ here:

cothread/src/cothread/catools.py

Lines 103 to 105 in bb62450

	def __bool__(self):
	return self.ok
	__nonzero__ = __bool__ # For python 2

but unfortunately this is a breaking change for any code which is testing bool(value) rather than value.ok. The only reasonable solution is probably to raise a one-shot deprecation warning when this function is called.

Fortunately I don't think the boolean test feature of ca_nothing is documented, but this still needs to be flagged as a breaking change.

[Araneidae]

Fortunately I don't think the boolean test feature of ca_nothing is documented, but this still needs to be flagged as a breaking change.

Unfortunately not only is this documented behaviour, it is recommended:

cothread/docs/user/catools.rst

Lines 218 to 223 in bb62450

	set. The return code can be tested for boolean success, so for example it
	is possible to write::
	if not caput(pv, value, throw=False):
	# process caput error

[Araneidae]

The patch described in DiamondLightSource/aioca#45 (comment) should probably be applied to cothread.