Fix in-tree PEP 517 backend wrapper example (#3893)

Author: abravalheri

changelog.d/3893.doc.rst

@@ -0,0 +1,3 @@
+Improved the documentation example regarding making a thin :pep:`517` in-tree
+backend wrapper of ``setuptools.build_meta`` that is future-proof and supports
+:pep:`660` hook too -- by :user:`webknjaz`.

docs/build_meta.rst

@@ -137,10 +137,7 @@ the ``_custom_build/backend.py`` file, as shown in the following example:
 .. code-block:: python
 
     from setuptools import build_meta as _orig
-
-    prepare_metadata_for_build_wheel = _orig.prepare_metadata_for_build_wheel
-    build_wheel = _orig.build_wheel
-    build_sdist = _orig.build_sdist
+    from setuptools.build_meta import *
 
 
     def get_requires_for_build_wheel(config_settings=None):
@@ -151,9 +148,15 @@ the ``_custom_build/backend.py`` file, as shown in the following example:
         return _orig.get_requires_for_build_sdist(config_settings) + [...]
 
 
-Note that you can override any of the functions specified in :pep:`PEP 517
-<517#build-backend-interface>`, not only the ones responsible for gathering
-requirements.
+.. note::
+
+   You can override any of the functions specified in :pep:`PEP 517
+   <517#build-backend-interface>`, not only the ones responsible for gathering
+   requirements. It is important to ``import *`` so that the hooks that you
+   choose not to reimplement would be inherited from the setuptools' backend
+   automatically. This will also cover hooks that might be added in the future
+   like the ones that :pep:`660` declares.
+
 
 .. important:: Make sure your backend script is included in the :doc:`source
    distribution </userguide/distribution>`, otherwise the build will fail.