Issue #25709: Fixed problem with in-place string concatenation and utf-8 cache.

diff --cc Misc/NEWS
index 6f86de701eb052f461c12e2bb353b94eecf77d9f,166864abdc9956aeae395128ebb19a71ef4840d9..2fbd20e50bd52c8b7a8ccc50ccf431832039ba8b
--- 1/Misc/NEWS
--- 2/Misc/NEWS
+++ b/Misc/NEWS
@@@ -10,11 -10,8 +10,13 @@@ Release date: XXXX-XX-X
  Core and Builtins
  -----------------
  
+ - Issue #25709: Fixed problem with in-place string concatenation and utf-8 cache.
+ 
 +- Issue #5319: New Py_FinalizeEx() API allowing Python to set an exit status
 +  of 120 on failure to flush buffered streams.
 +
 +- Issue #25485: telnetlib.Telnet is now a context manager.
 +
  - Issue #24097: Fixed crash in object.__reduce__() if slot name is freed inside
    __getattr__.
  

diff --cc Objects/unicodeobject.c
Simple merge