Is there a way to Create a Pseudo random without random.choice for strings?

I am trying to find a way around a problem in a project which sort of "Cripples" any random function but we are allowed to find a way around it and use that code. I am using strings for this.

I have tried multiple sources where I looked at seeds and randString.

    import random

    def randString(length=5):
       letters='rps'
       return ''.join((random.seed(letters) for i in range(length)))
    print ("Random String with specific letters ", randString() )
    print ("Random String with specific letters ", randString(5) )

I expect a random letter from 'rps' but due to the crippled random, I get repeated 'r'. ex.'rrrrrrrrrrrrrrr'