Revert "Switch to statistics.stdev"

This reverts commit 76df1dc56d.

Fix major performance regression

gptc/weighting.py

@@ -2,10 +2,45 @@
 
 import math
 from typing import Sequence, Union, Tuple, List
-import statistics
+
+
+def _mean(numbers: Sequence[float]) -> float:
+    """Calculate the mean of a group of numbers
+
+    Parameters
+    ----------
+    numbers : list of int or float
+        The numbers to calculate the mean of
+
+    Returns
+    -------
+    float
+        The mean of the numbers
+    """
+    return sum(numbers) / len(numbers)
+
+
+def _standard_deviation(numbers: Sequence[float]) -> float:
+    """Calculate the standard deviation of a group of numbers
+
+    Parameters
+    ----------
+    numbers : list of int or float
+        The numbers to calculate the mean of
+
+    Returns
+    -------
+    float
+        The standard deviation of the numbers
+
+    """
+    mean = _mean(numbers)
+    squared_deviations = [(mean - i) ** 2 for i in numbers]
+    return math.sqrt(_mean(squared_deviations))
 
 
 def weight(numbers: Sequence[float]) -> List[float]:
-    weight = statistics.stdev(numbers) * 2
+    standard_deviation = _standard_deviation(numbers)
+    weight = standard_deviation * 2
     weighted_numbers = [i * weight for i in numbers]
     return weighted_numbers