test_import: test_concurrency() leaks references and memory on Windows

[vstinner]

vstinner@WIN C:\victor\python\main>python -m test test_import -R 3:3 -v
(...)
test_import leaked [23, 23, 34] references, sum=80
test_import leaked [23, 21, 34] memory blocks, sum=78
test_import failed (reference leak) in 40.3 sec

test_concurrency() alone reproduces the issue:

vstinner@WIN C:\victor\python\main>python -m test test_import -R 3:3 -v -m test_concurrency 
(...)
test_import leaked [40, 40, 40] references, sum=120
test_import leaked [40, 40, 40] memory blocks, sum=120

---

By the way, test_typing also leaks:

vstinner@WIN C:\victor\python\main>python -m test test_typing -R 3:3
(...)
test_typing leaked [1, 1, 1] memory blocks, sum=3

[vstinner]

Note: On an old buildbot failure (19 days ago), I also saw a test_grammar leak, but I cannot reproduce it.

Build: https://buildbot.python.org/all/#/builders/920/builds/447

test_grammar leaked [2, 2, 2] references, sum=6
test_grammar leaked [2, 2, 2] memory blocks, sum=6

[vstinner]

Duplicate of #106176

I also found commit 3325f05 with a git bisect.