iter() and iterators

• iter(callable, sentinel) runs callable synchronously, so one that calls an external/OS function cannot suspend and raises NotImplementedError. Same limitation as map/filter/sorted(key=...).
• iter(callable, sentinel) compares result == sentinel, where CPython compares sentinel == result; only observable if the two sides have asymmetric __eq__.
• A StopIteration raised by callable propagates; CPython treats it as clean exhaustion and stops iterating.
• A user instance defining __call__ is rejected as not callable, since __call__ is not dispatched (see classes.md).
• The vendored type stub is upstream typeshed's verbatim, so -t accepts iter(obj) for an object with only __getitem__; Monty has no __getitem__ iteration fallback and raises TypeError at runtime.
• -t accepts for x in obj (though not a, b = obj) for a class that opts out of iteration with __iter__ = None, which raises TypeError at runtime as it does in CPython.
• Built-in iterators do not expose their dunders as attributes: hasattr(iter([1]), '__iter__') is False, where CPython reports True. Iteration itself works; only attribute lookup of the dunder differs. This covers every built-in iterator, including the itertools adaptors.
• Iterator-specific attributes such as __length_hint__ are not exposed.