Apply suggestions from code review

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Author: serhiy-storchaka

Lib/doctest.py

@@ -285,7 +285,7 @@ def getvalue(self):
         return result
 
     def truncate(self, size=None):
-        if size != None:
+        if size is not None:
             self.seek(size)
         StringIO.truncate(self)
 

Misc/NEWS.d/next/Library/2023-03-23-19-47-05.gh-issue-102967.gfIYkN.rst

@@ -1,2 +1,2 @@
-A bug in :func:`doctest._SpoofOut.truncate` was causing None to be passed to :func:`StringIO.seek` when no size was given.
-A simple fix skips the seek call when no size is given so the buffer can be truncated from the current position.
+A bug in :func:`!doctest._SpoofOut.truncate` was causing None to be passed to :func:`!StringIO.seek` when no size was given.
+A simple fix skips the seek call when no size is given so the buffer can be truncated from the current position.