Can you please attach a sample script to reproduce the behavior difference? With below script a dictionary is passed then dict.keys() supports subtraction over a list to return the difference. I am not sure how you have received a list with dict.keys() in Python 3.7.

import csv

fields = ["c", "d"]
with open('test.csv', 'w') as f:
    writer = csv.DictWriter(f, fieldnames=fields, extrasaction='raise')
    writer.writeheader()
    writer.writerow({"n": 1})