Fix python code style issue in linked list deque

[prinpal]

According to PEP 8, "Comparisons to singletons like None should always be done with is or is not, never the equality operators."

• This PR represents the translation of a single, complete document, or contains only bug fixes.
• The translation accurately conveys the original meaning and intent of the Chinese version. If deviations exist, I have provided explanatory comments to clarify the reasons.
• I have thoroughly reviewed the code, focusing on its formatting, comments, indentation, and file headers.
• I have confirmed that the code execution outputs are consistent with those produced by the reference code (Python or Java).
• The code is designed to be compatible on standard operating systems, including Windows, macOS, and Ubuntu.