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/Modules/_hashopenssl.c b/Modules/_hashopenssl.c
index d5dde6a..5b0a7be 100644
--- a/Modules/_hashopenssl.c
+++ b/Modules/_hashopenssl.c
@@ -706,7 +706,7 @@
     if (openssl_obj_name == NULL)
         return;
     /* Ignore aliased names, they pollute the list and OpenSSL appears to
-     * have a its own definition of alias as the resulting list still
+     * have its own definition of alias as the resulting list still
      * contains duplicate and alternate names for several algorithms.     */
     if (openssl_obj_name->alias)
         return;