AttributeError: module 'string' has no attribute 'uppercase'

[Anldra12]

If I understand your logic well, you are first adding all words to a dictionary. Then you are doing a lot of work to inspect the dictionary and move the words starting with a capital to the same word in lower case and delete the word starting with a capital.
My advice would be: make all keys lower case in the first place. Then the dictionary needs no post-processing.

    freq = {}
    for sentence in sentences:
        words = re.split(r"[^a-zA-Z0-9-]+", sentence)
        for word in words:
            count = freq.get(word.lower(), 0)
            freq[word.lower()] = count + 1