bpo-36389: Fix _PyBytesWriter in release mode (GH-16624)

Fix _PyBytesWriter API when Python is built in release mode with
assertions.

Include/internal/pycore_pymem.h

@@ -154,6 +154,21 @@ PyAPI_FUNC(int) _PyMem_SetDefaultAllocator(
     PyMemAllocatorDomain domain,
     PyMemAllocatorEx *old_alloc);
 
+/* Special bytes broadcast into debug memory blocks at appropriate times.
+   Strings of these are unlikely to be valid addresses, floats, ints or
+   7-bit ASCII.
+
+   - PYMEM_CLEANBYTE: clean (newly allocated) memory
+   - PYMEM_DEADBYTE dead (newly freed) memory
+   - PYMEM_FORBIDDENBYTE: untouchable bytes at each end of a block
+
+   Byte patterns 0xCB, 0xBB and 0xFB have been replaced with 0xCD, 0xDD and
+   0xFD to use the same values than Windows CRT debug malloc() and free().
+   If modified, _PyMem_IsPtrFreed() should be updated as well. */
+#define PYMEM_CLEANBYTE      0xCD
+#define PYMEM_DEADBYTE       0xDD
+#define PYMEM_FORBIDDENBYTE  0xFD
+
 /* Heuristic checking if a pointer value is newly allocated
    (uninitialized), newly freed or NULL (is equal to zero).