Currently, `inspect.signature` doesn't support all callables from the `operator` module, e.g. `operator.attrgetter`:

```python
>>> import inspect
>>> import operator
>>> inspect.signature(operator.attrgetter("spam"))
ValueError: callable operator.attrgetter('is_host') is not supported by signature
```

Support for this could be added either directly to `inspect.signature` or by adding `__signature__` attributes to `operator`'s classes.