Replace setup.py with pyproject.toml (#9021)

* Create pyproject.toml
* Implement a custom build backend (see below) in packager directory. Build logic from setup.py has been refactored and migrated into the new backend.
* Tested: pip wheel . (build wheel), python -m build --sdist . (source distribution)

doc/contrib/python_packaging.rst

@@ -0,0 +1,83 @@
+###########################################
+Notes on packaging XGBoost's Python package
+###########################################
+
+
+.. contents:: Contents
+  :local:
+
+.. _packaging_python_xgboost:
+
+***************************************************
+How to build binary wheels and source distributions
+***************************************************
+
+Wheels and source distributions (sdist for short) are the two main
+mechanisms for packaging and distributing Python packages.
+
+* A **source distribution** (sdist) is a tarball (``.tar.gz`` extension) that
+  contains the source code.
+* A **wheel** is a ZIP-compressed archive (with ``.whl`` extension)
+  representing a *built* distribution. Unlike an sdist, a wheel can contain
+  compiled components. The compiled components are compiled prior to distribution,
+  making it more convenient for end-users to install a wheel. Wheels containing
+  compiled components are referred to as **binary wheels**.
+
+See `Python Packaging User Guide <https://packaging.python.org/en/latest/>`_
+to learn more about how Python packages in general are packaged and
+distributed.
+
+For the remainder of this document, we will focus on packaging and
+distributing XGBoost.
+
+Building sdists
+===============
+
+In the case of XGBoost, an sdist contains both the Python code as well as
+the C++ code, so that the core part of XGBoost can be compiled into the
+shared libary ``libxgboost.so`` [#shared_lib_name]_.
+
+You can obtain an sdist as follows:
+
+.. code-block:: console
+
+  $ python -m build --sdist .
+
+(You'll need to install the ``build`` package first:
+``pip install build`` or ``conda install python-build``.)
+
+Running ``pip install`` with an sdist will launch CMake and a C++ compiler
+to compile the bundled C++ code into ``libxgboost.so``:
+
+.. code-block:: console
+
+  $ pip install -v xgboost-2.0.0.tar.gz  # Add -v to show build progress
+
+Building binary wheels
+======================
+
+You can also build a wheel as follows:
+
+.. code-block:: console
+
+   $ pip wheel --no-deps -v .
+
+Notably, the resulting wheel contains a copy of the shared library
+``libxgboost.so`` [#shared_lib_name]_. The wheel is a **binary wheel**,
+since it contains a compiled binary.
+
+
+Running ``pip install`` with the binary wheel will extract the content of
+the wheel into the current Python environment. Since the wheel already
+contains a pre-built copy of ``libxgboost.so``, it does not have to be
+built at the time of install. So ``pip install`` with the binary wheel
+completes quickly:
+
+.. code-block:: console
+  
+  $ pip install xgboost-2.0.0-py3-none-linux_x86_64.whl  # Completes quickly
+
+.. rubric:: Footnotes
+
+.. [#shared_lib_name] The name of the shared library file will differ
+   depending on the operating system in use. See :ref:`build_shared_lib`.