Fix python code style issue in linked list deque

[wodray]

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.
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• The code is designed to be compatible on standard operating systems, including Windows, macOS, and Ubuntu.