Made a straightforward patch for this. Probably not that pretty, so suggestions are welcome.

Note that some function names will become pretty long in exceptions:

>>> class A:
...     def __init__(self):
...         def f():
...             pass
...         f(1,2,3)
...
>>> A()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 5, in __init__
TypeError: A.__init__.<locals>.f() takes 0 positional arguments but 3 were given

Passing UTs from lastest snapshot.