## Summary
Implement Pylint rule `single-string-used-for-slots` (`C0205`) as
`single-string-slots` (`PLC0205`). This rule checks for single strings
being assigned to `__slots__`. For example
```python
class Foo:
__slots__: str = "bar"
def __init__(self, bar: str) -> None:
self.bar = bar
```
should be
```python
class Foo:
__slots__: tuple[str, ...] = ("bar",)
def __init__(self, bar: str) -> None:
self.bar = bar
```
Related to #970. Includes documentation.
## Test Plan
`cargo test`