Berker asks in IRC whether this change should go into 3.6.0 (at rc1).  While it is affecting a relatively self-contained part of the standard library (cgi), the issue doesn't seem to be "release critical".  Further, it is changing behavior that was changed barely a year ago for Issue24764.  My preference would be to try to have this change reviewed and/or tested by at least some of the people involved with the earlier issue and, if there is a consensus for it, target the change for 3.6.1.