explicitly declare setuptools as the build system

This is useful when installing on super minimal installs.  Then setuptools
is only installed when it is needed.

pyproject.toml

@@ -1,3 +1,7 @@
+
+[build-system]
+requires = ["setuptools"]
+
 # We ignore the following PEP8 warnings
 # * E123: closing bracket does not match indentation of opening bracket's line
 #   - Broken if multiple indentation levels start on a single line