[issue30936] json module ref leaks detected by test_json

[Segev Finer]

New submission from Segev Finer:

>From builtbots:

test_json leaked [4, 4, 4] references, sum=12
test_json leaked [3, 3, 3] memory blocks, sum=9
1 test failed again:
    test_json

Probably unearthed by:
https://github.com/python/cpython/pull/2692
https://github.com/python/cpython/pull/2693

----------
components: Extension Modules
messages: 298391
nosy: Segev Finer, haypo, serhiy.storchaka
priority: normal
severity: normal
status: open
title: json module ref leaks detected by test_json
type: resource usage
versions: Python 3.5, Python 3.6

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue30936>
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