Incorrect results for three-argument pow

Results from running Python on repl.it, which I'm assuming reflect here:

```
   pow(2,36,100000)
=> 69184L
   2**36 % 100000
=> 76736L
```

76736 is the correct result. Also:

```
   pow(2,40,100000)
=> 127776
```

which is greater than the passed modulus; it should be 27776.
