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.

Doc/tutorial/errors.rst

@@ -336,7 +336,7 @@ example::
 A *finally clause* is always executed before leaving the :keyword:`try`
 statement, whether an exception has occurred or not. When an exception has
 occurred in the :keyword:`try` clause and has not been handled by an
-:keyword:`except` clause (or it has occurred in a :keyword:`except` or
+:keyword:`except` clause (or it has occurred in an :keyword:`except` or
 :keyword:`else` clause), it is re-raised after the :keyword:`finally` clause has
 been executed.  The :keyword:`finally` clause is also executed "on the way out"
 when any other clause of the :keyword:`try` statement is left via a

Doc/tutorial/introduction.rst

@@ -303,7 +303,7 @@ For non-negative indices, the length of a slice is the difference of the
 indices, if both are within bounds.  For example, the length of ``word[1:3]`` is
 2.
 
-Attempting to use a index that is too large will result in an error::
+Attempting to use an index that is too large will result in an error::
 
    >>> word[42]  # the word only has 6 characters
    Traceback (most recent call last):