[`pylint`] Impement `self-assigning-variable` (`W0127`) (#6015)
## Summary
Implements Pylint rule [`self-assigning-variable`
(`W0127`)](https://pylint.pycqa.org/en/latest/user_guide/messages/warning/self-assigning-variable.html)
as `self-assigning-variable` (`PLW0127`). Includes documentation.
Related to #970.
## Test Plan
`cargo test`
