bpo-36326: Let inspect.getdoc() find docstrings for __slots__ (GH-12498)


diff --git a/Lib/statistics.py b/Lib/statistics.py
index e5a6246..bd8a6f9 100644
--- a/Lib/statistics.py
+++ b/Lib/statistics.py
@@ -709,7 +709,8 @@
     # https://en.wikipedia.org/wiki/Normal_distribution
     # https://en.wikipedia.org/wiki/Variance#Properties
 
-    __slots__ = ('mu', 'sigma')
+    __slots__ = {'mu': 'Arithmetic mean of a normal distribution',
+                 'sigma': 'Standard deviation of a normal distribution'}
 
     def __init__(self, mu=0.0, sigma=1.0):
         'NormalDist where mu is the mean and sigma is the standard deviation.'