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/codecs.py b/Lib/codecs.py
index 4a4d043..6cdf3f4 100644
--- a/Lib/codecs.py
+++ b/Lib/codecs.py
@@ -256,7 +256,7 @@
     """
     def __init__(self, errors='strict'):
         """
-        Create a IncrementalDecoder instance.
+        Create an IncrementalDecoder instance.
 
         The IncrementalDecoder may use different error handling schemes by
         providing the errors keyword argument. See the module docstring
@@ -1007,7 +1007,7 @@
     """
     Encoding iterator.
 
-    Encodes the input strings from the iterator using a IncrementalEncoder.
+    Encodes the input strings from the iterator using an IncrementalEncoder.
 
     errors and kwargs are passed through to the IncrementalEncoder
     constructor.
@@ -1025,7 +1025,7 @@
     """
     Decoding iterator.
 
-    Decodes the input strings from the iterator using a IncrementalDecoder.
+    Decodes the input strings from the iterator using an IncrementalDecoder.
 
     errors and kwargs are passed through to the IncrementalDecoder
     constructor.