Plug sys.exc_info() leaks

[tim-one]

BPO	473753
Nosy	@tim-one

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields: ```python assignee = None closed_at = created_at = labels = ['interpreter-core'] title = 'Plug sys.exc_info() leaks' updated_at = user = 'https://github.com/tim-one' ``` bugs.python.org fields: ```python activity = actor = 'jhylton' assignee = 'jhylton' closed = True closed_date = None closer = None components = ['Interpreter Core'] creation = creator = 'tim.peters' dependencies = [] files = [] hgrepos = [] issue_num = 473753 keywords = [] message_count = 2.0 messages = ['7121', '7122'] nosy_count = 2.0 nosy_names = ['tim.peters', 'jhylton'] pr_nums = [] priority = 'normal' resolution = 'fixed' stage = None status = 'closed' superseder = None type = None url = 'https://bugs.python.org/issue473753' versions = [] ```

[tim-one]

As the Library manual has warned for years, this kind of program leaks bigtime:

import sys
def f():
.    try:
.        1 / 0
.    except ZeroDivisionError:
.        err = sys.exc_info()

while 1:
.    f()

because err[-1] is a traceback object, from which f's frame can be reached, from which we can get back to 'err'.

2.2 added frames to gc (thanks to NeilS), but this example still leaks. That's at best an embarrassing trap, and it just popped up again in Zope's testing framework. Best guess is that a cure amounts to adding traceback objects to gc too, although I haven't tried that so am not sure.

[03bde425-37ce-4291-88bd-d6cecc46a30e]

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Fixed in rev 2.34 of traceback.c.