Python Primes 6n±1 Program

#!/usr/bin/python3
from math import sqrt, floor

# How many primes are 6n +/- 1?

total: int = 0
prime: int = 0

def is_prime(x: int) :
    for y in range(2, floor(sqrt(x) + 1)) :
        if x % y == 0 :
            return False
    return True

with open('prime_list.txt', 'w') as of :
    for x in range(1, 1000000) :
        for y in -1, 1 :
            z = 6 * x + y
            z1 = is_prime(z)
            total += 1
            prime += 1 if z1 else 0
            #
            print('6({:>10}){:+}={:>11} {} {:>4.15}%'.format(
                x,
                y,
                z,
                ('T' if z1 else 'F'),
                (prime * 100 / total)
            ), file=of)

input('Done.')
exit()