gh-104683: Argument clinic: remove the LandMine class

[AlexWaygood]

In Converter.__init__, self.function is temporarily set to be an instance of LandMine, for reasons explained in this comment:

cpython/Tools/clinic/clinic.py

Lines 2847 to 2853 in 6ef8f8c

	# this is deliberate, to prevent you from caching information
	# about the function in the init.
	# (that breaks if we get cloned.)
	# so after this change we will noisily fail.
	self.function: Function | LandMine = LandMine(
	"Don't access members of self.function inside converter_init!"
	)

However, this approach leads to some complicated typing elsewhere in clinic: there are several places where we had to add assert isinstance(self.function, Function) assertions in order to keep mypy happy when adding type annotations to clinic.py. An approach that works just as well, and makes mypy much more happy, is to delay setting self.function at all until after converter_init() has run. We can improve the error message if somebody does try to access self.function inside a converter_init() method by adding a __getattr__ method that catches the resulting AttributeError and adds to the error message the line number in the clinic input that caused clinic to fail.

• Issue: Modernise code in Tools/clinic/ #104683

[erlend-aasland]

Looks good to me; good riddance :)