Implemented with a bit of help from the Wikipedia page ;) Runs much faster than the naive approach.

SieveEratosthenes.py

def sieve(n):
    """Finds all prime numbers upto n
    """
    primelist = []
    intlist = list(range(int(n+1)))
    fin = int(n**0.5)
    
    for i in range(2,fin+1):
        intlist[2*i::i] = [None] * len(intlist[2*i::i])
        
    for item in intlist[2:]:
        if item: primelist.append(item)
    
    return primelist
    
#runtime calculation
import time
tStart=time.time()
sieve(6000)
print("Program complete")
print("Time taken:", time.time()-tStart)