recalculating only the new hanchans shoud do the trick now.

the racalc cronjob reports erronous partly recalculations now.
A lot of code cleanups

src/mahjong_ranking/tests.py

@@ -25,7 +25,7 @@ class KyuDanTest(TestCase):
         'test_kyu_dan_rankings.json'
     ]
 
-    def recalc(self):
+    def _test_recalc(self):
         """
         Test if a Kyu/Dan Ranking recalculation gives the same result as stored.
 
@@ -58,10 +58,10 @@ class KyuDanTest(TestCase):
                 user=ranking.user,
                 since=ranking.legacy_date
             )
+            if not confirmed_hanchans.count():
+                continue
             rnd = random.randrange(confirmed_hanchans.count())
             since = confirmed_hanchans[rnd].start
-            print(since)
-
             ranking.recalculate(hanchan_start=since)
             for attr in self.equal_attrs:
                 self.assertEqual(