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/difflib.py b/Lib/difflib.py
index 7eb42a9..56d4852 100644
--- a/Lib/difflib.py
+++ b/Lib/difflib.py
@@ -1469,7 +1469,7 @@
                 yield _make_line(lines,'-',0), None, True
                 continue
             elif s.startswith(('--?+', '--+', '- ')):
-                # in delete block and see a intraline change or unchanged line
+                # in delete block and see an intraline change or unchanged line
                 # coming: yield the delete line and then blanks
                 from_line,to_line = _make_line(lines,'-',0), None
                 num_blanks_to_yield,num_blanks_pending = num_blanks_pending-1,0