[`flake8-pyi`] Implement PYI017 (#5895)
## Summary
Implements `PYI017` or `Y017` from `flake8-pyi` plug-in. Mirrors
[upstream
implementation](https://github.com/PyCQA/flake8-pyi/blob/ceab86d16b822d12abae1d8087850d0bc66d2f52/pyi.py#L1039-L1048).
It checks for any assignment with more than 1 target or an assignment to
anything other than a name, and raises a violation for these in stub
files.
Couldn't find a clear and concise explanation for why this is to be
avoided and what is preferred for attribute cases like:
```python
a.b = int
```
So welcome some input there, to learn and to finish up the docs.
## Test Plan
Added test cases from upstream plug-in in a fixture (both `.py` and
`.pyi`). Added a few more.
## Issue link
Refers: https://github.com/astral-sh/ruff/issues/848
