Exercise: Named integers

The bool type produces True and False, which are something like named integers. The exercise is to produce, from something like a mapping, a subclass of int that provides named integers.

Just as True and False look nicer that 0 or 1 when a quantity is a boolean, so named integers look nicer with symbolic constants.

The interface should be something like

my_items = [(0, 'zero'), (1, 'one'), (2, 'two')]
OneTwoThree = whatsit(my_items)

The behavior should be something like

z = OneTwoThree('zero')
str(z) == 'zero'

t = OneTwoThree(2)
str(t) == 'two'

Thus, any string or integer (within range) can be used to produce a named integer. Out of range values should produce an exception.