Return NotImplemented from SortedSet set-algebra operators (#219)

[uttam12331]

Closes #219.

Problem

SortedSet.__and__, __or__, __sub__ and __xor__ were aliased directly to intersection/union/difference/symmetric_difference:

__and__ = intersection
__rand__ = __and__

So an unsupported (non-iterable) right-hand operand raised TypeError from deep inside the underlying set operation instead of returning NotImplemented. Per the data model, a binary operator should return NotImplemented for an operand type it doesn't support, letting Python try the other operand's reflected operator. Because SortedSet raised instead, this fell through:

class Interval:  # not iterable
    def __rand__(self, other):
        return SortedSet(x for x in other if ...)

sorted_set & interval   # TypeError, instead of using Interval.__rand__

The comparison operators (__eq__, __lt__, …) already handle this correctly via __make_cmp, which returns NotImplemented for unsupported operands — the set-algebra operators just hadn't been given the same treatment.

Change

Give each set-algebra operator an explicit implementation that returns NotImplemented when the operand is not iterable, and otherwise delegates to the existing method:

def __and__(self, other):
    if isinstance(other, Iterable):
        return self.intersection(other)
    return NotImplemented

__rand__ = __and__

This matches the approach @grantjenks outlined in the issue. Behavior is otherwise unchanged:

• Iterable operands (the existing, documented behavior — ss & [1, 2], ss - range(3), etc.) work exactly as before.
• A plain non-iterable operand with no reflected operator still ends up raising TypeError, just via Python's normal NotImplemented fallback rather than from inside the set operation.
• A non-iterable operand that defines a reflected operator now works (the motivating Interval.__rand__ case).

The named methods (intersection, union, difference, symmetric_difference) are untouched and still accept *iterables.

Tests

Added test_set_op_returns_notimplemented in tests/test_coverage_sortedset.py covering: the four dunders returning NotImplemented for a non-iterable, iterable operands still producing the right results, a reflected-operator type winning, and a plain non-iterable still raising TypeError. It fails on master (raises TypeError) and passes with this change. Full test_coverage_sortedset.py + test_stress_sortedset.py suite passes (53 passed); ruff check and ruff format --check are clean.