Fix SyntaxWarning from update message (#297)

The use of backslashes in the ASCII-art "Update" banner results in an "invalid escape sequence" SyntaxWarning under Python 3.12. Fix by changing the banner string to a raw string (which ignores backslashes).

Co-authored-by: Rosio <[email protected]>

nbclassic/notebookapp.py

@@ -325,7 +325,7 @@ def initialize_handlers(self):
         router.add_rules(core_rules)
         router.add_rules(static_handlers)
         router.add_rules(final_rules)
-        print("""
+        print(r"""
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