Proposed patch makes groupdict() without argument emitting FutureWarning if the result includes unmatched groups. Maybe warning message and the documentation could be better.

Ned, I ask your permission for including this change in 3.6. Yes, it can break third-party code that ran with -Werror, but I think groupdict() is rarely used in comparison of other match object API. The workaround is simple -- just use groupdict(None). The earlier we add a warning, the earlier we can change the behavior of groupdict(). If we did it in 3.4 (the time Raymond opened this issue), we could get new behavior in 3.7.