This is the shortest I could get.

def maxdepth(seq):
    """Find the maximum nesting level in seq
    In: list
    Out: int
    """
    if isinstance(seq, list):
        return 0 if not seq else max(maxdepth(seq[0]) + 1, maxdepth(seq[1:]))
    elif not isinstance(seq, list):
        return -1

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