bpo-39605: Remove a cast that causes a warning. (GH-18473)

diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index fdc2ca6..8470e41 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -3420,7 +3420,7 @@
 
     /* Decode via the codec registry */
     buffer = NULL;
-    if (PyBuffer_FillInfo(&info, NULL, (const void *)s, size, 1, PyBUF_FULL_RO) < 0)
+    if (PyBuffer_FillInfo(&info, NULL, (void *)s, size, 1, PyBUF_FULL_RO) < 0)
         goto onError;
     buffer = PyMemoryView_FromBuffer(&info);
     if (buffer == NULL)