class NameList(object):

    def __init__(self, names):
        self.names = names

def __contains__(self, name):
    return name.lower() in (n.lower() for n in self.names)

def add(self, name):
    self.names.append(name)
usernames = NameList(USERNAMES)
print(someone in usernames)
Нечувствительный к регистру «в» — Python