Issue #27942: String constants now interned recursively in tuples and frozensets.

diff --cc Misc/NEWS
index 8f60d8301bb65febe85384ce869f75be9f1a5ae2,5befca176c56cdf00ad004f06f9fd01579212691..4cdb356424a87f9da32a86d53dfc6d6415d4f660
--- 1/Misc/NEWS
--- 2/Misc/NEWS
+++ b/Misc/NEWS
@@@ -10,8 -10,8 +10,10 @@@ What's New in Python 3.7.0 alpha 
  Core and Builtins
  -----------------
  
+ - 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.