adding info on the required Numpy version to the recenlty added solution

Author: ibah

100 Numpy exercises.ipynb

@@ -1436,7 +1436,7 @@
    "outputs": [],
    "source": [
     "A = np.random.randint(0,10,(3,4,3,4))\n",
-	"# solution by passing a tuple of axes\n",
+	"# solution by passing a tuple of axes (introduced in numpy 1.7.0)\n",
 	"sum = A.sum(axis=(-2,-1))\n",
 	"print(sum)\n",
 	"# solution by flattening the last two dimensions into one\n",

100 Numpy exercises.md

@@ -693,7 +693,7 @@ print(np.unique(I))
 
 ```python
 A = np.random.randint(0,10,(3,4,3,4))
-# solution by passing a tuple of axes
+# solution by passing a tuple of axes (introduced in numpy 1.7.0)
 sum = A.sum(axis=(-2,-1))
 print(sum)
 # solution by flattening the last two dimensions into one