bpo-38356: Fix ThreadedChildWatcher thread leak in test_asyncio (GH-16552)

Motivation for this PR (comment from @vstinner in bpo issue):
```
Warning seen o AMD64 Ubuntu Shared 3.x buildbot:
https://buildbot.python.org/all/#/builders/141/builds/2593

test_devnull_output (test.test_a=syncio.test_subprocess.SubprocessThreadedWatcherTests) ...
Warning -- threading_cleanup() failed to cleanup 1 threads (count: 1, dangling: 2)
```
The following implementation details for the new method are TBD:

1) Public vs private

2) Inclusion in `close()`

3) Name

4) Coroutine vs subroutine method

5) *timeout* parameter

If it's a private method, 3, 4, and 5 are significantly less important.

I started with the most minimal implementation that fixes the dangling threads without modifying the regression tests, which I think is particularly important. I typically try to avoid directly modifying existing tests as much as possible unless it's necessary to do so. However, I am open to changing any part of this.


https://bugs.python.org/issue38356

Lib/asyncio/unix_events.py

@@ -1344,7 +1344,14 @@ class ThreadedChildWatcher(AbstractChildWatcher):
         return True
 
     def close(self):
-        pass
+        self._join_threads()
+
+    def _join_threads(self):
+        """Internal: Join all non-daemon threads"""
+        threads = [thread for thread in list(self._threads.values())
+                   if thread.is_alive() and not thread.daemon]
+        for thread in threads:
+            thread.join()
 
     def __enter__(self):
         return self