Change to hatchling build backend, and move to src layout

[mwtoews]

There are two key changes in this PR:

1. Change build backend from setuptools to hatchling. See "Why Hatch?" for some motivation.
2. Move source code to a src layout.

I've checked both sdist and bdist wheels, and the file structures are nearly identical. The primary "shapefile.py" still gets installed in the usual place.

[mwtoews]

Is there any reason why development isn't going to master? There are no CI tests to be configured on other branches.

[JamesParrott]

The CI should run on pushes to all branches (just not on PRs) I'm just keeping master on v2.4.0 until the publishing to PyPi process is fixed. It'll happen in a week or so.

[JamesParrott]

Thanks for this Mike. Hatchling is my personal preference also, so I have no objection. The src layout really is helpful too - it let me test a wheel (that I built from this PR without any problem) using test_shapefile.py, now that it's not in the same directory as shapefile.py.

But I'm going to hang back for now, to give other maintainers opportunity to object, and for the following additional reason:
currently the main CI test Custom Action only tests an editable install from a local repo.

I know it's only a single file library, and the build is all but trivial. But if we're going to change how our wheels are built, we should test the resulting wheels before shipping them. I don't think the tests will be noticeably slowed down if a build step is included before the editable install, and testing from a built wheel is more representative and reproducible.

Let me know if you want to tackle that, but I should remove the Python 2 mock server steps from this branch anyway so I'm happy to change the Action.

[Edit] Actually it's probably best if I do this, I'll change the action now on this branch. It's probably best, just in case it creates one of those tricky situations where the test workflow fails, but fails because it should fail (for a good reason, e.g. a bug elsewhere, not because there's anything wrong with the workflow).~~

[Edit 2] Change to Github Action now done.

[JamesParrott]

On reflection, this PR works nicely with the changes I've now made to the CI tests, and the ability to run the doctests from test_shapefile.py. Testing the wheels we actually ship, and clarity about what exactly the code under test is, are much more important than sticking to the tried and tested setup.py system for fear of breaking something. This is quite old fashioned now anyway, and a pyproject.toml /src layout is a great fit for the whole Python 3 modernisation effort.