Issue #21827: Fixed textwrap.dedent() for the case when largest common

whitespace is a substring of smallest leading whitespace.
Based on patch by Robert Li.

diff --cc Lib/test/test_textwrap.py
Simple merge

diff --cc Lib/textwrap.py
Simple merge

diff --cc Misc/ACKS
Simple merge

diff --cc Misc/NEWS
index 7e7534da5076b342a43c02c3f963c04d518dc805,3d4cbcb578bc4b123649762cc8642ea944e49322..638964ec3617f36cee7588d2e68f492bc16ed508
--- 1/Misc/NEWS
--- 2/Misc/NEWS
+++ b/Misc/NEWS
@@@ -45,10 -96,12 +45,14 @@@ Core and Builtin
  Library
  -------
  
 -- Issue #25471: Sockets returned from accept() shouldn't appear to be
 -  nonblocking.
+ - Issue #21827: Fixed textwrap.dedent() for the case when largest common
+   whitespace is a substring of smallest leading whitespace.
+   Based on patch by Robert Li.
+ 
 +- Issue #25447: The lru_cache() wrapper objects now can be copied and pickled
 +  (by returning the original object unchanged).
 +
 +- Issue #25390: typing: Don't crash on Union[str, Pattern].
  
  - Issue #25441: asyncio: Raise error from drain() when socket is closed.