Support for `__new__` constructor signatures

[jtbandes]

get_signatures seems to recognize def __init__(self, ...) methods, but does not recognize the def __new__(cls, ...) static method:

>>> jedi.Script("class C:\n  def __init__(self, a: int):\n    pass\nC(").get_signatures(4,2)
[<Signature: index=0 C(a: int)>]

>>> jedi.Script("class C:\n  def __new__(cls, a: int):\n    pass\nC(").get_signatures(4,2)
[<Signature: index=None C()>]

I would expect the a= param to be present in this signature. For example, ipython3 recognizes the constructor call and offers the param for tab-completion: