🐍 Use the stdlib for our score statistics (#20)

Python 3.4 [introduced](https://docs.python.org/3/library/statistics.html)
the `statistics` module into the standard library. Let's use that
instead of calculating our own simply for the sake of not reinventing
the wheel.

grants/models.py

@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+from statistics import mean, variance, stdev
+
 from django.db import models
 from urlman import Urls
 
@@ -141,24 +143,19 @@ def average_score(self):
         scores = [s.score for s in self.scores.all() if s.score]
         if not scores:
             return None
-        else:
-            return sum(scores) / float(len(scores))
+        return mean(scores)
 
     def variance(self):
         data = [s.score for s in self.scores.all() if s.score]
-        n = len(data)
-        if n == 0:
-            return 0
-        c = sum(data) / float(len(data))
-        if n < 2:
+        if not data:
             return 0
-        ss = sum((x - c) ** 2 for x in data)
-        ss -= sum((x - c) for x in data) ** 2 / len(data)
-        assert not ss < 0, "negative sum of square deviations: %f" % ss
-        return ss / (n - 1)
+        return variance(data)
 
     def stdev(self):
-        return self.variance() ** 0.5
+        data = [s.score for s in self.scores.all() if s.score]
+        if not data:
+            return 0
+        return stdev(data)
 
 
 class Allocation(models.Model):