Improve code with Python 3.x-only features

[StephenChan]

Go through Python 3.x changelogs for new features to use sooner rather than later. Prominent examples:

• 3.1 simplified super() (done in PR #383)
• 3.2 TemporaryDirectory to improve our unit test framework with regards to media cleanup
• 3.3 list copy() instead of [:]
• 3.3 revisit our circular import workarounds since import behavior has changed
• 3.3 datetime.datetime.timestamp()
• 3.4 enums, and 3.6 enum.auto
• 3.4 TestCase.subTest()
• 3.5 type hints

Prerequisite: #59, and getting all systems onto Python 3.x. Related: #364 (cleaning up 2.x compatibility code)