Monty doesn't accept `slice()` arguments outside i64 range

[davidhewitt]

On CPython the slice constructor can accept any values, e.g.

>>> slice('foo')
slice(None, 'foo', None)

It is only the .indices() operation that can fail on non-integer types:

>>> slice('foo').indices(10)
Traceback (most recent call last):
  File "<python-input-1>", line 1, in <module>
    slice('foo').indices(10)
    ~~~~~~~~~~~~~~~~~~~~^^^^
TypeError: slice indices must be integers or None or have an __index__ method

Currently monty will fail to construct slice('foo'). We should decide if we care.