Use assert_never

[JPHutchins]

Now that TypeIs is in use, exhaustive type checking can be asserted at compile time instead of run time; for example, here:

smpclient/examples/ble/imagestate.py

Lines 26 to 31 in 83cfd1c

	if success(response):
	print(f"Received response: {response}")
	elif error(response):
	print(f"Received error: {response}")
	else:
	raise Exception(f"Unknown response: {response}")

The examples should demonstrate this approach.