Issue #25523: Correct "a" article to "an" article

This changes the main documentation, doc strings, source code comments, and a
couple error messages in the test suite. In some cases the word was removed
or edited some other way to fix the grammar.
diff --git a/Lib/test/pystone.py b/Lib/test/pystone.py
index a41f1e5..59dd99b 100755
--- a/Lib/test/pystone.py
+++ b/Lib/test/pystone.py
@@ -35,7 +35,7 @@
                 Under Python 3 version 1.1 would use the normal division
                 operator, resulting in some of the operations mistakenly
                 yielding floats. Version 1.2 instead uses floor division
-                making the benchmark a integer benchmark again.
+                making the benchmark an integer benchmark again.
 
 """