This will be much faster. It does fewer prime checks and does not check numbers that are even for prime. 

**With GMPY2:**

 $ time python3 goldbach.py 12345678912345678
 This is a test of Goldbach's Conjecture that for all even integers
 greater than 2 there are two primes that add up to that even number.
 
 12,345,678,912,345,678 = 61 + 12,345,678,912,345,617
 
 real	0m0.027s
 user	0m0.018s
 sys	0m0.009s

**With my Home Grown IsPrime:**

 $ time python3 goldbach.py 12345678912345678
 This is a test of Goldbach's Conjecture that for all even integers
 greater than 2 there are two primes that add up to that even number.
 
 12,345,678,912,345,678 = 61 + 12,345,678,912,345,617
 
 real	0m2.424s
 user	0m2.420s
 sys	0m0.004s

**Listing:**

 # Goldbach's Conjecture tester. 
 from gmpy2 import is_prime
 import sys
 # or use home grown
 def IsPrime(n): 
 if (n == 2 or n == 3): 
 return True 
 if (n <= 1 or n % 2 == 0 or n % 3 == 0): 
 return False 
 for i in range(5, int(n**.5)+1, 6): 
 if (n % i == 0 or n % (i + 2) == 0): 
 return False 
 return True 
 numb = int(sys.argv[1])
 print("This is a test of Goldbach's Conjecture that for all even integers")
 print("greater than 2 there are two primes that add up to that even number.\n")
 while (numb < 3 or numb%2):
 	numb = int(input("Please enter an even number > 3 to check with Goldbach's Conjecture> "))
 if numb == 4:
 	print(f" 4 = 2 + 2") # the only time we use 2
 else:
 	for p in range(3, numb, 2): # just check odds after 2 
 		if IsPrime(p) and IsPrime(numb-p):
 			print(f" {numb:,} = {p:,} + {numb-p:,}")
 			break`enter code here`