TestExceptionInfo.test_traceback_not_referenced fails with pypy3

[mgorny]

Describe the bug
TestExceptionInfo.test_traceback_not_referenced fails when the test suite is run using PyPy3:

_________________________________________________ TestExceptionInfo.test_traceback_not_referenced _________________________________________________

self = <test_errors.TestExceptionInfo object at 0x00007fb05c3fbb40>

    def test_traceback_not_referenced(self):
        """
        Ensure that an ExceptionInfo does not keep a reference
        to the traceback, as doing so can create unintended
        side effects. See #386 for more info.
        """
        state = mock.MagicMock()
        _ = errors.ExceptionInfo(None, None, DeletionDetector(state))
>       assert state.deleted
E       AssertionError: assert False
E        +  where False = <MagicMock id='140395438949168'>.deleted

tests/test_errors.py:24: AssertionError

To Reproduce
Steps to reproduce the behavior:

1. Install dependencies for pypy3 (tox doesn't seem to work here, I used distribution packages)
2. pypy3 -m pytest

Expected behavior
All tests passing.

Environment

• OS: Gentoo Linux

$ pypy3 --version
Python 3.6.9 (?, Apr 10 2020, 20:05:19)
[PyPy 7.3.1 with GCC 9.2.0]

Run from git master (fb74a37).

Additional context
My guess would be that it's due to different GC behavior in PyPy.

[jaraco]

`tox` doesn't seem to work here

This finding was a surprise to me. So it seems that neither black no mypy install on pypy (due to python/typed_ast#111). I've reached out to the typing team on gitter to see how permanent that condition is.

[jaraco]

I've merged 15f6272, which addresses the tox issue.