This PR has introduced reference leaks. Bisecting points to:

fe13f0b0f696464dd6f283576668dbf57cb11399 is the first bad commit
commit fe13f0b0f696464dd6f283576668dbf57cb11399
Author: Yurii Karabas <1998uriyyo@gmail.com>
Date:   Fri Jul 23 12:47:00 2021 +0300

    bpo-44676: Add ability to serialize types.Union (GH-27244)

 Lib/test/test_types.py                             | 35 +++++++++++++++
 Lib/typing.py                                      |  4 +-
 .../2021-07-19-19-53-46.bpo-44676.WgIMvh.rst       |  2 +
 Objects/unionobject.c                              | 51 ++++++++++++++++++++++
 4 files changed, 90 insertions(+), 2 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Core and Builtins/2021-07-19-19-53-46.bpo-44676.WgIMvh.rst
bisect run successfe13f0b0f696464dd6f283576668dbf57cb11399 is the first bad commit
commit fe13f0b0f696464dd6f283576668dbf57cb11399
Author: Yurii Karabas <1998uriyyo@gmail.com>
Date:   Fri Jul 23 12:47:00 2021 +0300

    bpo-44676: Add ability to serialize types.Union (GH-27244)

 Lib/test/test_types.py                             | 35 +++++++++++++++
 Lib/typing.py                                      |  4 +-
 .../2021-07-19-19-53-46.bpo-44676.WgIMvh.rst       |  2 +
 Objects/unionobject.c                              | 51 ++++++++++++++++++++++
 4 files changed, 90 insertions(+), 2 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Core and Builtins/2021-07-19-19-53-46.bpo-44676.WgIMvh.rst
bisect run success

Example failure:

https://buildbot.python.org/all/#/builders/259/builds/100

Ran 106 tests in 0.053s
OK
......
test_types leaked [2, 2, 2] references, sum=6
test_types leaked [1, 1, 1] memory blocks, sum=3
1 test failed again:
    test_types
== Tests result: FAILURE then FAILURE ==
413 tests OK.