Commit 9a4135e939bc223f592045a38e0f927ba170da32 introduced a reference leak:

https://buildbot.python.org/all/#/builders/80/builds/587/steps/3/logs/stdio


Bisect results for test_future:

0:00:00 load avg: 10.04 [1/1] test_future
beginning 9 repetitions
123456789
.........
test_future leaked [24, 24, 24, 24] references, sum=96
test_future leaked [24, 24, 24, 24] memory blocks, sum=96
test_future failed

== Tests result: FAILURE ==

1 test failed:
    test_future

Total duration: 300 ms
Tests result: FAILURE
9a4135e939bc223f592045a38e0f927ba170da32 is the first bad commit
commit 9a4135e939bc223f592045a38e0f927ba170da32
Author: Eric V. Smith <ericvsmith@users.noreply.github.com>
Date:   Wed May 8 16:28:48 2019 -0400

    bpo-36817: Add f-string debugging using '='. (GH-13123)

    If a "=" is specified a the end of an f-string expression, the f-string will evaluate to the text of the expression, followed by '=', followed by the repr of the value of the expression.

:040000 040000 303c86dc65bb09cade6b8f2a0fa3f97715f1793b 7ddfc1c6c5f86bf6d0a38a64ff1415c7ca55a5fe M      Doc
:040000 040000 88a8dc9ffb46d652d086168e18e6a1265e0bfb92 847032e70ebd74ad7e828f8013acc7bdc4570779 M      Include
:040000 040000 119e11a4f2fa3966ca300d06dd44c46dcbae11a8 ed8c093aecd82a6b886d83218681df886f653206 M      Lib
:040000 040000 9c8d303bc3b468a06c39389520795bd34cfc7534 36b368a8e4ddd3f470c99b68b00a4a3a30c8c2b1 M      Misc
:040000 040000 ac4c322bf5eb2685c31b375bb8a7d49a7b6a9ed9 8e311dcfcafb1260eb5c617783318088a029b067 M      Parser
:040000 040000 e9db8405b51b7f72015f70556ce611c371450731 9c3da4ade5f48b3a2e842d845cafa58ba21089a3 M      Python
bisect run success