Fix Python 2 issue in take-nth

It was trying to `return` a value in a generator, which Python 2 forbids.

hy/core/language.hy

@@ -419,14 +419,13 @@
 (defn take-nth [n coll]
   "Return every nth member of coll
      raises ValueError for (not (pos? n))"
-  (if (pos? n)
-    (do
+  (if (not (pos? n))
+    (raise (ValueError "n must be positive")))
   (setv citer (iter coll) skip (dec n))
   (for* [val citer]
     (yield val)
     (for* [_ (range skip)]
       (next citer))))
-    (raise (ValueError "n must be positive"))))
 
 (defn zero? [n]
   "Return true if n is 0"