redeclared-assigned-name: Allow dummy variables instead of hard-coded underscore

[richardebeling]

The check for redeclared-assigned-name has an exception hard-coded for _. If _ can not be used for unpacking return values, this is inconvenient. The check should instead allow all values that match the dummy-variables-rgx configuration.

Steps to reproduce

def some_method():
    return 1, 2, 3

_unused, a, _unused = some_method()

Run pylint --disable=all --enable=redeclared-assigned-name --dummy-variables-rgx='_unused' sample.py

Current behavior

************* Module sample
sample.py:4:0: W0128: Redeclared variable '_unused' in assignment (redeclared-assigned-name)

Expected behavior

no errors. _unused is a valid dummy variable name.

pylint --version output

pylint 2.4.4
astroid 2.3.3
Python 3.7.5 (default, Nov  7 2019, 10:50:52) 
[GCC 8.3.0]

but the hard coded exception for "_" is still in master, so I don't see why any newer version would behave differently.