v0.16.31

fiss.py:

• attr_delete now works on pairs
• mop now recurses into arrays and complex data structures to discover files referenced by attributes
• attr_get and attr_list are now reference data aware (matching Terra UI), and can return or elide them from results.

Dockerfile:

• base image changed to Python 3

__init__.py:

• Suppress Python 2 deprecation warning from cryptography module.