Workaround for #9149 (#12050)

var/spack/repos/builtin/packages/py-numpy/package.py

@@ -201,6 +201,9 @@ def build_args(self, spec, prefix):
         return args
 
     def setup_environment(self, spack_env, run_env):
+        # If py-numpy is installed as an external package, python won't
+        # be available in the spec. See #9149 for details.
+        if 'python' in self.spec:
             python_version = self.spec['python'].version.up_to(2)
 
             include_path = join_path(