Clarify modules used for pyproject.toml parsing are private (#3385)

changelog.d/3385.misc.rst

@@ -0,0 +1,2 @@
+Modules used to parse and evaluate configuration from ``pyproject.toml`` files are
+intended for internal use only and that not part of the public API.

setuptools/config/_apply_pyprojecttoml.py

@@ -4,6 +4,8 @@
 The distribution and metadata objects are modeled after (an old version of)
 core metadata, therefore configs in the format specified for ``pyproject.toml``
 need to be processed before being applied.
+
+**PRIVATE MODULE**: API reserved for setuptools internal usage only.
 """
 import logging
 import os

setuptools/config/expand.py

@@ -14,6 +14,8 @@
 
 This module focus on the second step, and therefore allow sharing the expansion
 functions among several configuration file formats.
+
+**PRIVATE MODULE**: API reserved for setuptools internal usage only.
 """
 import ast
 import importlib

setuptools/config/pyprojecttoml.py

@@ -1,4 +1,8 @@
-"""Load setuptools configuration from ``pyproject.toml`` files"""
+"""
+Load setuptools configuration from ``pyproject.toml`` files.
+
+**PRIVATE MODULE**: API reserved for setuptools internal usage only.
+"""
 import logging
 import os
 import warnings

setuptools/config/setupcfg.py

@@ -1,4 +1,8 @@
-"""Load setuptools configuration from ``setup.cfg`` files"""
+"""
+Load setuptools configuration from ``setup.cfg`` files.
+
+**API will be made private in the future**
+"""
 import os
 
 import warnings