update sentences2vec function for gensim 4.0

[Malledive]

def sentences2vec(sentences, model, unseen=None):
    """Generate vectors for each sentence (list) in a list of sentences. Vector is simply a
    sum of vectors for individual words.
    
    Parameters
    ----------
    sentences : list, array
        List with sentences
    model : word2vec.Word2Vec
        Gensim word2vec model
    unseen : None, str
        Keyword for unseen words. If None, those words are skipped.
        https://stats.stackexchange.com/questions/163005/how-to-set-the-dictionary-for-text-analysis-using-neural-networks/163032#163032

    Returns
    -------
    np.array
    """
    
    keys = set(model.wv.key_to_index)
    vec = []
    
    if unseen:
        unseen_vec = model.wv.get_vector(unseen)

    for sentence in sentences:
        if unseen:
            vec.append(sum([model.wv.get_vector(y) if y in set(sentence) & keys
                       else unseen_vec for y in sentence]))
        else:
            vec.append(sum([model.wv.get_vector(y) for y in sentence 
                            if y in set(sentence) & keys]))
    return np.array(vec)```