[BUG] syntax error in implementation of filter_non_unique

[geekypandey]

A small syntax issue in the implementation of filter_non_unique.

from collections import Counter

def filter_non_unique(lst):
    return [item for item, count in counter = Counter(lst).items() if count == 1]

I think we could implement the similar thing in python3.8 using the Walrus operator ( :=).