bpo-42195: Disallow isinstance/issubclass for subclasses of genericaliases in Union...

bpo-42195: Disallow isinstance/issubclass for subclasses of genericaliases in Union (GH-24059)

Previously this didn't raise an error. Now it will:
```python
from collections.abc import Callable
isinstance(int, list | Callable[..., str])
```
Also added tests in Union since there were previously none for stuff like ``isinstance(list, list | list[int])`` either.

Backport to 3.9 not required.

Automerge-Triggered-By: GH:gvanrossum