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

[saimn]

Basically src-layout makes it much easier to test the installed version of a package (and thus to test that the package has been packaged/installed correctly), by ensuring that the package cannot be imported from the local directory. With a flat-layout it can be very difficult to do so, because amongst other of pytest’s way to discover conftest.py and test files.

More info on src-layout: https://blog.ionelmc.ro/2014/05/25/python-packaging/#the-structure Also ref https://github.com/astropy/package-template/issues/522

For astropy the current way to test the installed package only works because pytest-astropy-header is importing astropy (without needing it) before pytest runs the conftest and test discovery (!!): https://github.com/astropy/pytest-astropy-header/issues/41 (especially https://github.com/astropy/pytest-astropy-header/issues/41#issuecomment-1002705819 which summarizes the thing …) https://github.com/astropy/pytest-astropy-header/pull/43 (nice fix 😄)

I lost hours on this testing the installed package thing for various packages, it’s often painful to do unless you use a src-layout … (for flat-layout it depends if the test files are in the package or not, if using conftest.py in the root directory, and probably other things I forgot). So yeah src-layout may look weird at first but for my mental health it’s now my preferred choice 😉.

[pllim]

But, we have so many open PRs and stuff… This is something that needs to be coordinated well to happen without making everyone else cry.