FIX: atexit hook to clean up cache threads, TyphosSuite garbage collection

[klauer]

Description

• Add cleanup for background threads
• Add replacement for functools.partial usage in methods as this was causing TyphosSuite from getting garbage collected

Motivation and Context

• Dangling cache daemon threads appear to cause a segfault on pip-installed PyQt5 but not conda-forge PyQt5
• I tried the signal QApplication.aboutToQuit instead of atexit, thinking it'd be better to clean up when the QApplication does. I found that the slot wasn't getting called and fell back to atexit.
• The garbage collection issue led to the test suite failing in a very strange way:
  • The benchmarks in test_benchmark are configured to run and create very nested device displays. These device displays were supposed to get garbage collected between tests, but they were not.
  • Once the test suite reached test_cli, typhos instructed Qt to reload stylesheets for every widget in the application.
  • This caused massive amounts of recursive updates of style sheets on a ton (?) of widgets from every previous test. This bogged the test suite down so much that it used to take over 20 minutes to run. This proposal should hopefully fix that.

How Has This Been Tested?

• Running the lightpath test suite in a Docker container closely resembling Travis CI (Temporary: local docker container for testing lightpath#156)
• And the test suite itself

Where Has This Been Documented?

PRs and issues

[tangkong]

This works for me inside a docker container that reproduces travis testing conditions, removing the QThread teardown failures at the end of the pytest suite. Other test suite bugs are still in travis however

[ZLLentz]

I think I need to do my homework and finally read up on how all the weakref mechanisms work

All of this looks good to me

[tangkong]

This looks good to me. The test suite passes on Travis, so I'd say the proof is in the pudding.

This left me wondering what other apps might benefit from garbage collection utilities like this. I suppose we won't really know until we hit upon more problems, but if it's been solved once it can be solved again!

[tangkong]

To make sure I have this straight, TyphosBase._connect_partial connects a WeakPartialMethodSlot to a signal.

• Should we name _connect_partial -> _weak_connect_partial or something that makes it obvious that weak refs are being used? I can see myself forgetting that and thinking _connect_partial is just a wrapper around functools.partial

[klauer]

Tweaked the name - going to merge and re-run CI on another long-open PR