Fix pickle py2/3 incompatibility in node attrs.
Fix pickle py2/3 incompatibility in node attrs.

Pickle is incompatible between python 2 and 3. Numpy takes care of
most of the incompatibilities (numpy>=1.9) but (for example) datetime
objects can still produce UnicodeDecodeErrors when pickle on python 2
and unpickled on python 3.
In python 3.4 encoding='bytes' was added to fix this.
http://bugs.python.org/issue6784

FrancescAlted/PyTables,PyTables/PyTables,avalentino/PyTables,FrancescAlted/PyTables,FrancescAlted/PyTables,avalentino/PyTables,PyTables/PyTables,avalentino/PyTables,PyTables/PyTables