Add assert to rangesum JUUUUUUUST in case

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

day07/day07.py

@@ -4,6 +4,7 @@ from typing import Sequence
 
 
 def rangesum(rn: range):
+    assert rn.step == 1, "this function doesn't work when range step != 1"
     return (((rn.stop + 1) * rn.stop) // 2) - (((rn.start + 1) * rn.start) // 2)