Install setuptools 'core' extra

Under as-yet-unidentified conditions, we can end up with a version of
packaging that is too old for the version of latest version of
setuptools. This is a known issue and expected behavior and per [1]
$subject is the preferred resolution.

[1] https://github.com/pypa/setuptools/issues/4483#issuecomment-2237219597

Change-Id: I9232f3fae1598297e83c4ea37339896f7dcbd44f
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>
diff --git a/lib/infra b/lib/infra
index 2aad003..f4760c3 100644
--- a/lib/infra
+++ b/lib/infra
@@ -31,7 +31,7 @@
     local PIP_VIRTUAL_ENV="$REQUIREMENTS_DIR/.venv"
     [ ! -d $PIP_VIRTUAL_ENV ] && ${VIRTUALENV_CMD} $PIP_VIRTUAL_ENV
     # We don't care about testing git pbr in the requirements venv.
-    PIP_VIRTUAL_ENV=$PIP_VIRTUAL_ENV pip_install -U pbr setuptools
+    PIP_VIRTUAL_ENV=$PIP_VIRTUAL_ENV pip_install -U pbr setuptools[core]
     PIP_VIRTUAL_ENV=$PIP_VIRTUAL_ENV pip_install $REQUIREMENTS_DIR
 
     # Unset the PIP_VIRTUAL_ENV so that PBR does not end up trapped