asyncio, Tulip issue 137: In debug mode, save traceback where Future, Task and
Handle objects are created. Pass the traceback to call_exception_handler() in
the 'source_traceback' key.

The traceback is truncated to hide internal calls in asyncio, show only the
traceback from user code.

Add tests for the new source_traceback, and a test for the 'Future/Task
exception was never retrieved' log.
Fix indentation and class name in socket howto example.
Closes #21582: Cleanup test_asyncore. Patch written by diana.

- Use support.captured_stderr() where appropriate
- Removes some "from test.support import xxx" import and uses support.xxx
  instead.
Issue #11453: asyncore: emit a ResourceWarning when an unclosed file_wrapper
object is destroyed. The destructor now closes the file if needed. The close()
method can now be called twice: the second call does nothing.
asyncio: Fix unit tests on Windows, escape filenames in regex
asyncio: Fix two "Coroutine xxx was never yielded from" messages in tests