Do not raise unused-argument for singledispatch implementations when the argument is used in another implementation

[anntzer]

Steps to reproduce

In

from functools import singledispatch

@singledispatch
def func(arg):
    return arg

@func.register(int)
def _(arg):
    return 42

print(func(1), func("foo"))

pylint warns that the implementation of func for int does not use argument arg. But similarly to #225, it typically makes sense to just maintain the same signature for all implementations.

Current behavior

warning

Expected behavior

no warning

pylint --version output

pylint 1.6.4,
astroid 1.4.7
Python 3.5.2 (default, Jun 28 2016, 08:46:01)
[GCC 6.1.1 20160602]

[PCManticore]

This is fixed now in the master branch.