add changelog entry

Author: benoit-pierre

changelog.d/3421.breaking.rst

@@ -0,0 +1,4 @@
+Drop setuptools' support for installing an entrypoint extra requirements at load time:
+- the functionality has been broken since v60.8.0.
+- the mechanism to do so is deprecated (`fetch_build_eggs`).
+- that use case (e.g. a custom command class entrypoint) is covered by making sure the necessary build requirements are declared.