Remove the 'enth_type' attribute used by H5DictNode and H5TableNode

It's a bit too magic, and it creates backwards compatibility problems.

This was causing trouble in some product code which used an earlier version of this code. The attribute name `enth_type` was changed from a product-specific name. That causes problems when reading old data with this code. It turns out that checking for a child node named `_pyobject_data` in the group created by `H5DictNode` is a sufficient test.