(Merge 3.4) 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.

diff --cc Lib/asyncore.py
Simple merge

diff --cc Lib/test/test_asyncore.py
Simple merge

diff --cc Misc/NEWS
Simple merge