feat: PEP 723 for python examples

Adding a [PEP 723](https://peps.python.org/pep-0723/) header to the python example files would make them self-contained and executable by any PEP 723-compatible tool, like `uv` or `hatch`. If there's interest in this feature I'm happy to open a PR. We would add this comment to the top of every example in `python/examples/`:

```python
# /// script
# requires-python = ">=3.10"
# dependencies = [
# "neuroglancer",
# "numpy",
# ]
# ///
```