issue2551232 - modify in-reply-to threading when multiple matches if an email is missing an issue designator, in-reply-to threading is attempted. In this change if in-reply-to threading matches multiple issues, fall back to matching on subject. It used to just arbitrairly choose the first matching issue.

try:
    # Python 3+
    from ssl import SSLError
except (ImportError, AttributeError):
    # Python 2.5-2.7
    from socket import sslerror as SSLError