Fix ruff linting warnings from generated template files for extension modules (#10371)

## Summary

This PR fixes two ruff linting issues in the generated template files
when using: `uv init --build-backend` for extension modules.

1. Removes unnecessary `from __future__ import annotations` imports from
generated .pyi files
([PYI044](https://docs.astral.sh/ruff/rules/future-annotations-in-stub/))
2. Adds missing blank line after `hello_from_bin` import to comply with
isort formatting
([I001](https://docs.astral.sh/ruff/rules/unsorted-imports/))

## Test Plan

```bash
cargo run -- init --build-backend scikit-build-core example-ext
uvx ruff check example-ext --select ALL

cargo run -- init --build-backend maturin example-ext
uvx ruff check example-ext --select ALL
```

## Remaining warnings

There are still warnings remainings in the generated `__init__.py`
files:
- [D104](https://docs.astral.sh/ruff/rules/undocumented-public-package/)
Missing docstring in public package
-
[D103](https://docs.astral.sh/ruff/rules/undocumented-public-function/)
Missing docstring in public function
- [T201](https://docs.astral.sh/ruff/rules/print/) `print` found

docs/concepts/projects/init.md

@@ -279,6 +279,7 @@ And the Python module imports it:
 ```python title="src/example_ext/__init__.py"
 from example_ext._core import hello_from_bin
 
+
 def main() -> None:
     print(hello_from_bin())
 ```