scons package: add missing dependency to py-setuptools (#10856)

var/spack/repos/builtin/packages/scons/package.py

@@ -19,3 +19,10 @@ class Scons(PythonPackage):
 
     # Python 3 support was added in SCons 3.0.0
     depends_on('python@:2', when='@:2', type=('build', 'run'))
+    depends_on('py-setuptools', when='@3.0.2:', type='build')
+
+    # Prevent passing --single-version-externally-managed to
+    # setup.py, which it does not support.
+    @when('@3.0.2:')
+    def install_args(self, spec, prefix):
+        return ['--prefix={0}'.format(prefix), '--root=/']