Fix check unused arguments false positive bug

Problem: the special method `__new__` must match the arguments of the
`__init__` method even if `__new__` method does not use them. This
generate `unused-argument` for the `__new__` method.

Fix: the unused arguments check should not be done on the `__new__`
method if the `__init__` method is defined in the same class.

Fixes pylint-dev#3670

Author: Emplis

doc/whatsnew/fragments/3670.false_positive

@@ -0,0 +1,3 @@
+Fix `unused-argument` false positive when `__new__` does not use all the arguments of `__init__`.
+
+Closes #3670

pylint/checkers/variables.py

@@ -2593,6 +2593,16 @@ def _check_is_unused(
         argnames = node.argnames()
         # Care about functions with unknown argument (builtins)
         if name in argnames:
+            if node.name == "__new__":
+                is_init_def = False
+                # Look for the `__init__` method in all the methods of the same class.
+                for n in node.parent.get_children():
+                    is_init_def = hasattr(n, "name") and (n.name == "__init__")
+                    if is_init_def:
+                        break
+                # Ignore unused arguments check for `__new__` if `__init__` is defined.
+                if is_init_def:
+                    return
             self._check_unused_arguments(name, node, stmt, argnames, nonlocal_names)
         else:
             if stmt.parent and isinstance(