Allow execution of pyw files on Unix (#9759)
I don't see any real reason to forbid executing these in a
cross-platform way
```
❯ echo "print('hello world')" > test.pyw
❯ uv run test.pyw
error: Failed to spawn: `test.pyw`
Caused by: No such file or directory (os error 2)
❯ cargo run -q -- run test.pyw
hello world
```
Closes https://github.com/astral-sh/uv/issues/9757
