Use pyproject.toml with hatchling instead of setuptools

The main motivation for this change is to migrate from the non-standard `setup.py` to the standard PEP 517 `setup.py`. The secondary motivation is to use a build backend that's easier to use and harder to get wrong than setuptools.

Hatchling is the most popular build backend behind setuptools (I can provide data if required), and much easier to use than setuptools. Notably, it will include `py.typed` and `.pyi` files automatically. This also applies to most other build backends. This means we can remove the setuptools-specific documentation.

packaging.python.org has a tab system to show different build backends (https://packaging.python.org/en/latest/guides/writing-pyproject-toml/). Afaik typing.python.org doesn't, so we only show one build backend.

Fixes #2121

Author: konstin

docs/guides/libraries.rst

@@ -82,25 +82,25 @@ A typical directory structure would look like:
 
 .. code-block:: text
 
-   setup.py
+   pyproject.toml
    my_great_package/
       __init__.py
       stuff.py
       py.typed
 
 It's important to ensure that the ``py.typed`` marker file is included in the
-distributed package. If using ``setuptools``, this can be achieved like so:
+distributed package. If using ``hatchling``, it is included by default:
 
-.. code-block:: python
+.. code-block:: toml
 
-   from setuptools import setup
+   [project]
+   name = "my-great-package"
+   version = "0.1.0"
+   requires-python = ">=3.14"
 
-   setup(
-      name="my_great_distribution",
-      version="0.1",
-      package_data={"my_great_package": ["py.typed"]},
-      packages=["my_great_package"],
-   )
+   [build-system]
+   requires = ["hatchling"]
+   build-backend = "hatchling.build"
 
 
 Type stub files included in the package
@@ -113,27 +113,13 @@ directory structure would look like:
 
 .. code-block:: text
 
-   setup.py
+   pyproject.toml
    my_great_package/
       __init__.py
       stuff.py
       stuff.pyi
       py.typed
 
-If using ``setuptools``, we can ensure the ``.pyi`` and ``py.typed`` files are
-included like so:
-
-.. code-block:: python
-
-   from setuptools import setup
-
-   setup(
-      name="my_great_distribution",
-      version="0.1",
-      package_data={"my_great_package": ["py.typed", "stuff.pyi"]},
-      packages=["my_great_package"],
-   )
-
 The presence of ``.pyi`` files does not affect the Python interpreter at runtime
 in any way. However, static type checkers will only look at the ``.pyi`` file and
 ignore the corresponding ``.py`` file.
@@ -150,41 +136,28 @@ For example:
 
 .. code-block:: text
 
-   setup.py
+   pyproject.toml
    my_great_package-stubs/
       __init__.pyi
       stuff.pyi
 
+.. code-block:: toml
 
-.. code-block:: python
-
-   from setuptools import setup
+   [project]
+   name = "my-great-package-stubs"
+   version = "0.1.0"
+   requires-python = ">=3.14"
 
-   setup(
-      name="my_great_package-stubs",
-      version="0.1",
-      package_data={"my_great_package-stubs": ["__init__.pyi", "stuff.pyi"]},
-      packages=["my_great_package-stubs"]
-   )
+   [build-system]
+   requires = ["hatchling"]
+   build-backend = "hatchling.build"
 
+   [tool.hatch.build.targets.wheel]
+   packages = ["src/my_great_package-stubs"]
 
 Users are then able to install the stubs-only package separately to provide
 types for the original library.
 
-Inclusion in sdist
-^^^^^^^^^^^^^^^^^^
-
-Note that to ensure inclusion of ``.pyi`` and ``py.typed`` files in an sdist
-(.tar.gz archive), you may also need to modify the inclusion rules in your
-``MANIFEST.in`` (see the
-`packaging guide <https://packaging.python.org/en/latest/guides/using-manifest-in/>`__
-for more details on ``MANIFEST.in``). For example:
-
-.. code-block:: text
-
-   global-include *.pyi
-   global-include py.typed
-
 .. _type_completeness:
 
 How much of my library needs types?