Issue #3080: Import builtins using Unicode strings

- is_builtin(), init_builtin(), load_builtin() and other builtin related
   functions use Unicode strings, instead of byte strings
 - Rename _PyImport_FixupExtensionUnicode() to _PyImport_FixupExtensionObject()
 - Rename _PyImport_FindExtensionUnicode() to _PyImport_FindExtensionObject()