Move astropy to ``src`` directory

### Description

Having the following package structure is considered much safer.

```
astropy/
├── pyproject.toml
├── ...
├── src/
│   └── astropy/
```



### Additional context

https://packaging.python.org/en/latest/tutorials/packaging-projects/#creating-the-package-files
https://setuptools.pypa.io/en/latest/userguide/package_discovery.html