Handle type guards properly in Receiver.filter()

[llucax]

Now the Receiver type returned by Receiver.filter() will have the narrowed type when a TypeGuard is used.

[shsms]

You don't need to change _Filter because TypeGuard is just another name for bool, with some additional semantic meaning. The type system knows assigning a TypeGuard value to a bool is fine.

Suggested change

	# We need to use Any here because otherwise _Filter would have to deal with two
	# different signatures. We can create two filter classes, one for regular functions
	# and one for type guards, but then there is no way to tell at runtime which
	# function is a type guard and which isn't to instantiate the correct class.
	# Using Any here has no impact though, as thanks to the overloads, only the
	# overloaded types will be accepted.
	def filter(
	self, filter_function: Callable[[ReceiverMessageT_co], Any], /
	) -> Receiver[Any]:
	def filter(
	self,
	filter_function: (
	Callable[[ReceiverMessageT_co], bool]
	| Callable[[ReceiverMessageT_co], TypeGuard[FilteredMessageT_co]]
	),
	/,
	) -> Receiver[ReceiverMessageT_co] | Receiver[FilteredMessageT_co]: