merge 3.6 (#26617)

diff --cc Misc/NEWS
index 5b6ed49bcd303885a7682a763851b43352a01738,121e567891725e546ff8b2d1d6c1aa6e54cf32d1..0fea54780aa14fb37a62d463c6f72ad988be38eb
--- 1/Misc/NEWS
--- 2/Misc/NEWS
+++ b/Misc/NEWS
@@@ -10,10 -10,10 +10,12 @@@ What's New in Python 3.7.0 alpha 
  Core and Builtins
  -----------------
  
+ - Issue #26617: Fix crash when GC runs during weakref callbacks.
+ 
  - Issue #27942: String constants now interned recursively in tuples and frozensets.
  
 +- Issue #28289: ImportError.__init__ now resets not specified attributes.
 +
  - Issue #21578: Fixed misleading error message when ImportError called with
    invalid keyword args.