add alias for hostpython/python, that point to either 2 or 3 version depending of the previous compiled state.

This allows the recipe to point on an versionless python.

recipes/python.py

@@ -0,0 +1,15 @@
+# -*- coding: utf-8 -*-
+from toolchain import Recipe
+
+class PythonAliasRecipe(Recipe):
+    is_alias = True
+
+    def init_after_import(self, ctx):
+        python = ctx.state.get("python")
+        if not python:
+            print("WARNING: no python set yet, so you need to specify")
+            print("WARNING: either python2 or python3 in your deps")
+        else:
+            self.depends = [python]
+
+recipe = PythonAliasRecipe()