Test attached unittest_leak.py script: you will see MyException.ninstance counter increased up to 10, whereas I expect that MyException is destroyed at TestCase.run() exit.

It looks like a tricky reference cycle between:

- frames
- exc_info local variable of _Outcome.testPartExecutor() context manager
- _Outcome.errors list
- _Outclass instance

Attached unittest_workaround.patch patch works around the issue.