Make it clear that copy_reg.pickle() should not be used for classes, but only for extension types.
Make it clear that copy_reg.pickle() should not be used for classes, but
only for extension types.

This partially fixes SourceForge bug #116295.

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator