Fix serialisation when constructor has py3 type annotations

[mbatchkarov]

By using getfullargspec instead of the deprecated getargspec. Classes like this now work:

class Person:
     def __init__(self, name: str):
         self.name = name

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PS I'm running Python 3.6. This won't work on 2.7 as it is, but it can be done. How important is it to support 2.7?