Allow execution of pyw files on Unix

[zanieb]

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