Update code to ensure tests pass correctly in more modern verisons of python

[MoralCode]

This PR attempts to fix the unit tests in the cleanest way possible with minimal changes to existing functionality.

Changes include:

• sqlite databases used for tests are now saved to the output directory with the generated images (instead of in-memory) to allow failed tests to be more easily debugged
• some fixes were made to attachments.py to correct an issue where thumbnails were not being correctly saved to an Image object, causing test failures (fixes #133)
• more assertions were added to better detect the previous issue
• some tweaks were mane to the Image unit tests to ensure that objects from the previous session weren't being reused in a new session (seemingly intended to simulate retrieval from the DB at a later time) and causing sqlalchemy to crash
• small tweaks to variable names in image tests to make the intention clearer and prevent potential issues with variable shadowing
• corrected typos in file and class naming for the GoogleCloudStorage class to allow the test to properly load
• updated mocks and the file paths to the stuff/cat.jpg image in GoogleCloudStorage tests so the tests actually run