Markov Analysis not working

[5Head]

Write a program to read a text from a file and perform Markov analysis. The result should be
a dictionary that maps from prefixes to a collection of possible suffixes. The collection might
be a list, tuple, or dictionary; it is up to you to make an appropriate choice. You can test your
program with prefix length two, but you should write the program in a way that makes it easy
to try other lengths.

Here's what I have so far:

import string
def word_dict(f,length): # takes length and performs markov analysis, returns dictionary 
    fin=open(f,encoding='utf-8') 
    l=((fin.read()).strip(string.punctuation).split(' ')) 
    d={}
    for i in range(len(l)-length-1): # adds combined strings to dictionary, then assigns the word following it to the key
        s=' '.join(l[i:i+length])
        d.setdefault(s,[]).append(l[i+length]);
    return d

print(word_dict('words.txt',3))]

It's not working correctly.

Here is the file: http://gutenberg.org/files/63437/63437-0.txt