SF bug #761337: datetime.strftime fails on trivial format string

The interning of short strings violates the refcnt==1 assumption for
_PyString_Resize().

A simple fix is to boost the initial value of "totalnew" by 1.
Combined with an NULL argument to PyString_FromStringAndSize(),
this assures that resulting format string is not interned.
This will remain true even if the implementation of
PyString_FromStringAndSize() changes because only the uninitialized
strings that can be interned are those of zero length.

Added a test case.
diff --git a/Lib/test/test_datetime.py b/Lib/test/test_datetime.py
index cca0c9d..c6dbb48 100644
--- a/Lib/test/test_datetime.py
+++ b/Lib/test/test_datetime.py
@@ -831,6 +831,7 @@
     def test_strftime(self):
         t = self.theclass(2005, 3, 2)
         self.assertEqual(t.strftime("m:%m d:%d y:%y"), "m:03 d:02 y:05")
+        self.assertEqual(t.strftime(""), "") # SF bug #761337
 
         self.assertRaises(TypeError, t.strftime) # needs an arg
         self.assertRaises(TypeError, t.strftime, "one", "two") # too many args