When I commented code to hide logs in the tests, I saw another bug.

Local patch:

diff --git a/Lib/test/test_asyncio/test_subprocess.py b/Lib/test/test_asyncio/test_subprocess.py       
index 177a02cdcc..3095b1d987 100644
--- a/Lib/test/test_asyncio/test_subprocess.py
+++ b/Lib/test/test_asyncio/test_subprocess.py
@@ -456,7 +456,8 @@ class SubprocessMixin:

         # ignore the log:
         # "Exception during subprocess creation, kill the subprocess"
-        with test_utils.disable_logger():
+        #with test_utils.disable_logger():
+        if 1:
             self.loop.run_until_complete(cancel_make_transport())
             test_utils.run_briefly(self.loop)


Output:

0:00:05 [ 14] test_asyncio
test_cancel_post_init (test.test_asyncio.test_subprocess.SubprocessProactorTests) ...

Exception in call
back _ProactorReadPipeTransport._loop_reading(<Future finished result=b''>)
handle: <Handle _ProactorReadPipeTransport._loop_reading(<Future finished result=b''>)>
Traceback (most recent call last):
  File "C:\vstinner\python\master\lib\asyncio\proactor_events.py", line 293, in _loop_reading
    data = self._data[:length]
TypeError: slice indices must be integers or None or have an __index__ method

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\vstinner\python\master\lib\asyncio\events.py", line 80, in _run
    self._context.run(self._callback, *self._args)
  File "C:\vstinner\python\master\lib\asyncio\proactor_events.py", line 325, in _loop_reading
    if length > -1:
TypeError: '>' not supported between instances of 'bytes' and 'int'


Exception in callback _ProactorReadPipeTransport._loop_reading(<Future finished result=b''>)
handle: <Handle _ProactorReadPipeTransport._loop_reading(<Future finished result=b''>)>
Traceback (most recent call last):
  File "C:\vstinner\python\master\lib\asyncio\proactor_events.py", line 293, in _loop_reading
    data = self._data[:length]
TypeError: slice indices must be integers or None or have an __index__ method

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\vstinner\python\master\lib\asyncio\events.py", line 80, in _run
    self._context.run(self._callback, *self._args)
  File "C:\vstinner\python\master\lib\asyncio\proactor_events.py", line 325, in _loop_reading
    if length > -1:
TypeError: '>' not supported between instances of 'bytes' and 'int'
ok