bpo-37158: Simplify and speed-up statistics.fmean() (GH-13832)

Lib/statistics.py

@@ -320,11 +320,11 @@ def fmean(data):
     except TypeError:
         # Handle iterators that do not define __len__().
         n = 0
-        def count(x):
+        def count(iterable):
             nonlocal n
-            n += 1
-            return x
-        total = fsum(map(count, data))
+            for n, x in enumerate(iterable, start=1):
+                yield x
+        total = fsum(count(data))
     else:
         total = fsum(data)
     try: