Revert "Remove useless code."

This reverts commit 02c6e63ac8dbb8acde3bc19abd36fff0db65fd11. Apparently
types derived from containers can have a container base class without
the container class having a metaclass.

This fixes the sample_strlist unit test crashing. (There may be another
bug here, in Shiboken's don't-write-generated-source-if-not-changed
logic, as the failure doesn't seem to reliably appear/disappear after an
incremental build.)

Change-Id: I5f60bfb784dca02d9766a32e7b7b8a8a529df5af
Reviewed-by: John Ehresman <jpe@wingware.com>