singly_linked_list.py: psf/black (#2008)

* singly_linked_list.py: psf/black

* updating DIRECTORY.md

* Update singly_linked_list.py

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>

data_structures/linked_list/singly_linked_list.py

@@ -1,9 +1,9 @@
-class Node:  # create a Node
+class Node:
     def __init__(self, data):
-        self.data = data  # given data
-        self.next = None  # given next to None
+        self.data = data
+        self.next = None
 
-    def __repr__(self):  # string representation of a Node
+    def __repr__(self):
         return f"Node({self.data})"