It looks like the test if not the fix for #15578 reveals a reference leak somewhere in the import system:

$ ./python Lib/test/regrtest.py -R :: test_import
[1/1] test_import
beginning 9 repetitions
123456789
.........
test_import leaked [1, 1, 1, 1] references, sum=4
Warning -- sys.path was modified by test_import
1 test failed:
    test_import
[57131 refs]