Actually fix negative check

vm/src/builtins/int.rs

@@ -425,10 +425,13 @@ impl PyInt {
                 if modulus.is_zero() {
                     return Err(vm.new_value_error("pow() 3rd argument cannot be 0".to_owned()));
                 }
-                if modulus.is_negative() {
-                    return Err(vm.new_not_implemented_error("modular inverses".to_owned()));
-                }
-                self.general_op(other, |a, b| Ok(vm.ctx.new_int(a.modpow(b, modulus))), vm)
+                self.general_op(other, |a, b| {
+                    if b.is_negative() {
+                        Err(vm.new_not_implemented_error("modular inverses".to_owned()))
+                    } else {
+                        Ok(vm.ctx.new_int(a.modpow(b, modulus)))
+                    }
+                }, vm)
             }
             None => self.general_op(other, |a, b| inner_pow(a, b, vm), vm),
         }