Introduce a new Arg node for Python 3 function arguments

[pylint-bot]

Originally reported by: Claudiu Popa (BitBucket: PCManticore, GitHub: @PCManticore?)

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We currently don't have an Arg node for Python 3's arguments, because we are using AssName instead (this is how it works on Python 2).

#!python

>>> from ast import parse
>>> x = parse('''
... def test(a): pass
... ''')
>>> print(x)
<_ast.Module object at 0x0000000DBEF6F908>
>>> x.body
[<_ast.FunctionDef object at 0x0000000DBEF6F940>]
>>> x.body[0]
<_ast.FunctionDef object at 0x0000000DBEF6F940>
>>> x.body[0].args
<_ast.arguments object at 0x0000000DBEF6FA20>
>>> x.body[0].args.args
[<_ast.arg object at 0x0000000DBEF76A58>]
>>>

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• Bitbucket: https://bitbucket.org/logilab/astroid/issue/134

[pylint-bot]

Original comment by Claudiu Popa (BitBucket: PCManticore, GitHub: @PCManticore?):

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In fact, it's nicer to have AssName instead of a new Arg node, since it needs to be the same on both Python 2 and Python 3. Introducing a new node will change that requirement.