Set _PYTHON_ECLASS_INHERITED.
(Patch by Arfrever. Backported from Progress Overlay.)
--- python.eclass
+++ python.eclass
@@ -9,6 +9,8 @@
 # @DESCRIPTION:
 # The python eclass contains miscellaneous, useful functions for Python packages.
 
+_PYTHON_ECLASS_INHERITED="1"
+
 inherit multilib
 
 if ! has "${EAPI:-0}" 0 1 2 3; then