Replace IndexedGzipFile.__reduce__ with IndexedGzipFile.__reduce_ex__

Restores “pickle-ability” of IndexedGzipFile in Python 3.12, which was
broken due to python/cpython#101948.

indexed_gzip/indexed_gzip.pyx

@@ -169,9 +169,11 @@ class IndexedGzipFile(io.BufferedReader):
             return self.read(nbytes)
 
 
-    def __reduce__(self):
+    def __reduce_ex__(self, protocol):
         """Used to pickle an ``IndexedGzipFile``.
 
+        :arg protocol:         Pickle protocol version
+
         Returns a tuple containing:
           - a reference to the ``unpickle`` function
           - a tuple containing a "state" object, which can be passed
@@ -1093,7 +1095,7 @@ def unpickle(state):
     """Create a new ``IndexedGzipFile`` from a pickled state.
 
     :arg state: State of a pickled object, as returned by the
-                ``IndexedGzipFile.__reduce__`` method.
+                ``IndexedGzipFile.__reduce_ex__`` method.
 
     :returns:   A new ``IndexedGzipFile`` object.
     """