Moved 20 to 26 to Python.

python/e021.py

@@ -0,0 +1,15 @@
+from lib_misc import sum_proper_divisors
+
+
+def euler_021():
+    t = 0
+    for n in range(1, 10000):
+        s = sum_proper_divisors(n)
+        if n != s and n == sum_proper_divisors(s):
+            t += n
+    return t
+
+
+if __name__ == "__main__":
+    assert(euler_021() == 31626)
+    print("e021.py: {}".format(euler_021()))