save the __builtin__ module in a static variable.  But this doesn't
work across Py_Finalise()/Py_Initialize()!  It also doesn't work when
using multiple interpreter states created with PyInterpreterState_New().

So I'm ripping out this small optimization.

This was probably broken since PyImport_Import() was introduced in
1997!  We really need a better test suite for multiple interpreter
states and repeatedly initializing.

This fixes the problems Barry reported in Demo/embed/loop.c.