fix: Provide a sequence to random.sample

The sample function used to automatically convert sets to sequences but
that is no longer supported starting in 3.11 so we have to do it
manually.

Reference: https://docs.python.org/3/library/random.html#random.sample

xmodule/library_content_block.py

@@ -233,7 +233,7 @@ def make_selection(cls, selected, children, max_count, mode):
             pool = valid_block_keys - selected_keys
             if mode == "random":
                 num_to_add = min(len(pool), num_to_add)
-                added_block_keys = set(rand.sample(pool, num_to_add))
+                added_block_keys = set(rand.sample(list(pool), num_to_add))
                 # We now have the correct n random children to show for this user.
             else:
                 raise NotImplementedError("Unsupported mode.")