pyproject.toml: require package pipcl for building.

Author: julian-smith-artifex-com

pyproject.toml

@@ -1,8 +1,4 @@
 [build-system]
-# We define required packages in setup.py:get_requires_for_build_wheel().
-requires = []
-
-# See pep-517.
-#
-build-backend = "setup"
-backend-path = ["."]
+    requires = ['pipcl']
+    build-backend = "setup"
+    backend-path = ["."]