This now leaves `inspect.getsource` inconsistent with `inspect.getsourcefile`:

>>> import inspect
>>> from contextlib import contextmanager

>>> @contextmanager
... def func():
...    yield

>>> inspect.getsource(func)
'@contextmanager\ndef func():\n    yield\n'
>>>inspect.getsourcefile(func)
'C:\\Program Files\\Python 3.5\\lib\\contextlib.py'

Should `getsourcefile` be changed to match?

This is causing numpy/numpy#11639, but it's not clear if this is a bug or by design.