command.build_py have spec only in py3

Author: tharvik

stdlib/2and3/distutils/command/build_py.pyi

@@ -1,6 +1,8 @@
 # Stubs for distutils.command.bdist_msi
 
 from ..cmd import Command
+import sys
 
-class build_py(Command): ...
-class build_py_2to3(Command): ...
+if sys.version_info >= (3,):
+    class build_py(Command): ...
+    class build_py_2to3(Command): ...