Fix a few different issues that I noticed while working on #103:
The PYTEST_ADDOPTS environment variable wasn't being passed through to pytest by tox, apparently due to our config file not using a wildcard where we meant to.
Tox wasn't exercising Python 3.11 or 3.12 by default (even though our CI exercised them)
One of our tests would fail if run in isolation due to libpthread "leaking" memory by saving a thread and its stack for later reuse.
Fix a few different issues that I noticed while working on #103:
PYTEST_ADDOPTS
environment variable wasn't being passed through to pytest by tox, apparently due to our config file not using a wildcard where we meant to.