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There is a newer version: 4.23.0.17664

Private attributes which are written but never read are a case of "dead store". Changing their value is useless and most probably indicates an
error in the code.
Why is this an issue?
Python has no real private attribute. Every attribute is accessible. There are however two conventions indicating that an attribute is not meant to
be "public":

   attributes with a name starting with a single underscore (ex: _myattribute) should be seen as non-public and might change without
  prior notice. They should not be used by third-party libraries or software. It is ok to use those methods inside the library defining them but it
  should be done with caution. 
   "class-private" attributes have a name starting with at least two underscores and ending with at most one underscore. These attributes' names
  will be automatically mangled to avoid collision with subclasses' attributes. For example __myattribute will be renamed as
  _classname__myattribute, where classname is the attribute’s class name without its leading underscore(s). They shouldn’t
  be used outside of the class defining the attribute. 

This rule raises an issue when a class-private attribute (two leading underscores, max one underscore at the end) is never read inside the class.
It optionally raises an issue on unread attributes prefixed with a single underscore. Both class attributes and instance attributes will raise an
issue.
How to fix it
Remove the attribute or fix the code to read it.
Code examples
Noncompliant code example
class Noncompliant:
    _class_attr = 0  # Noncompliant if enable_single_underscore_issues is enabled
    __mangled_class_attr = 1  # Noncompliant

    def __init__(self, value):
        self._attr = 0  # Noncompliant if enable_single_underscore_issues is enabled
        self.__mangled_attr = 1  # Noncompliant

    def compute(self, x):
        return x * x

Compliant solution
class Compliant:
    _class_attr = 0
    __mangled_class_attr = 1

    def __init__(self, value):
        self._attr = 0
        self.__mangled_attr = 1

    def compute(self, x):
        return x * Compliant._class_attr * Compliant.__mangled_class_attr * self._attr * self.__mangled_attr

Resources
Documentation

   Python documentation – Private Variables 
   PEP 8 – Style Guide for Python Code