Update fill_textbox to better respect rect.width

The function norm_words in fill_textbox had a bug in its last loop, appending n+1 characters when actually measuring width of n characters.
It led to a bug in fill_texbox when you tried to write a single word mostly composed of "wide" letters (M,m, W, w...), causing the written text to exceed the given rect.

The fix was just to replace n+1 by n.

fitz/utils.py

@@ -4356,10 +4356,10 @@ def norm_words(width, words):
             while n > 0:
                 wl = sum(wl_lst[:n])
                 if wl <= width:
-                    nwords.append(w[: n + 1])
+                    nwords.append(w[:n])
                     word_lengths.append(wl)
-                    w = w[n + 1 :]
-                    wl_lst = wl_lst[n + 1 :]
+                    w = w[n:]
+                    wl_lst = wl_lst[n:]
                     n = len(wl_lst)
                 else:
                     n -= 1