How do I get the constructor signature for built-in types?

[Steven D'Aprano]

The inspect.getargspec and getfullargspec functions allow you to extract
the function call signature for Python functions and methods. This allows
you to work out the constructor signature for pure-Python classes, by
calling inspect.getargspec on the __init__ or __new__ method.

.... def __init__(self, a, b, *args):
.... pass
....ArgSpec(args=['self', 'a', 'b'], varargs='args', keywords=None,
defaults=None)



So far so good. But if the __init__ method is inherited directly from a
built-in, getargspec fails:

.... pass
....Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/usr/local/lib/python3.2/inspect.py", line 794, in getargspec
getfullargspec(func)
File "/usr/local/lib/python3.2/inspect.py", line 821, in getfullargspec
raise TypeError('{!r} is not a Python function'.format(func))
TypeError: <slot wrapper '__init__' of 'object' objects> is not a Python
function


How do I programmatically get the argument spec of built-in types'
__init__ or __new__ methods?