Hello. I investigated ref leak report related to thread.
Please run python regrtest.py -R :: test_leak.py (attached file)
Sometimes ref leak is reported.
# I saw this as regression failure on python-checkins.

# total ref count 92578 -> 92669
  _Condition 2
  Thread 6
  _Event 1
  bool 10
  instancemethod 1
  code 2
  dict 9
  file 1
  frame 3
  function 2
  int 1
  list 2
  builtin_function_or_method 5
  NoneType 2
  str 27
  thread.lock 7
  tuple 5
  type 5

Probably this happens because threading.Thread is implemented as Python
code,
(expecially threading.Thread#join), the code of regrtest.py

        if i >= nwarmup:
            deltas.append(sys.gettotalrefcount() - rc - 2)

can run before thread really quits. (before Moudles/threadmodule.c
t_bootstrap()'s

 Py_DECREF(boot->func);
 Py_DECREF(boot->args);
 Py_XDECREF(boot->keyw);

runs)

So I experimentally inserted the code to wait for thread termination.
(attached file experimental.patch) And I confirmed error was gone.

# Sorry for hackish patch which only runs on windows. It should run
# on other platforms if you replace Sleep() in Python/sysmodule.c
# sys_debug_ref_leak_leave() with appropriate function.