How to combine dictionary of word-forms into a single list without duplicates
You can get a list of all possible words (including the nouns, verbs, adjectives and so on) by using a comprehension. This loops over all elements of a sequence, as with a for-loop, but can be faster.

In you case, you have a dictionary, so we loop over each key-value pair. The keys are n (noun), a (adjective), v (verb) and r (??). For each of these keys, we have a list of words, which we can append to your final desired output list, if they aren't already in the list.
Because we loop over a dictionary, and then over each of the lists, with becomes nested:

from word_forms.word_forms import get_word_forms
word_forms = get_word_forms("review")

desired_words = []
for word_type, word_list in word_forms.items():
    for word in word_list:
        if word not in desired_words:
            desired_words.append(word)

print(desired_words)
# ['reviewers', 'review', 'reviewer', 'reviews', 'reviewing', 'reviewed']


This can be compressed into a nested comprehension:

desired_words = []
{desired_words.append(word) for word_type, word_list in word_forms.items() for word in word_list if word not in desired_words}

print(desired_words)
# ['reviewers', 'review', 'reviewer', 'reviews', 'reviewing', 'reviewed']


We didn't need to save the resulting object, we only want to side-effect of appending to desired_words

Another way to do this would be to simply put all the words into a list, then convert it into a set() object, which would remove all duplicates and order the list alphabetically:

desired_words = []
# the 'if' statement is removed in the dictionary comprehension:
{desired.words.append(word) for word_type, word_list in word_forms.items() for word in word_list}
# Set removes duplicates - we then wrap it back into a list
desired_words = list(set(desired_words))