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
to fix the grammar.
diff --git a/Lib/test/test_os.py b/Lib/test/test_os.py
index 57c7c88..fb34893 100644
--- a/Lib/test/test_os.py
+++ b/Lib/test/test_os.py
@@ -649,7 +649,7 @@
         except OSError as e:
             self.assertEqual(e.errno, errno.EBADF)
         else:
-            self.fail("%r didn't raise a OSError with a bad file descriptor"
+            self.fail("%r didn't raise an OSError with a bad file descriptor"
                       % f)
 
     @unittest.skipUnless(hasattr(os, 'isatty'), 'test needs os.isatty()')