keepalive: implement _close_conn() so closes are known Keepalives were not working on Python 3 because http.client.HTTPResponse was refactored to call _close_conn() instead of close(). Our custom close() is what returns inactive connections to the available state. We better support Python 3 by implementing a _close_conn(). Differential Revision: https://phab.mercurial-scm.org/D5720