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.

Lib/asyncio/streams.py

@@ -255,7 +255,7 @@ class StreamWriter:
     def __init__(self, transport, protocol, reader, loop):
         self._transport = transport
         self._protocol = protocol
-        # drain() expects that the reader has a exception() method
+        # drain() expects that the reader has an exception() method
         assert reader is None or isinstance(reader, StreamReader)
         self._reader = reader
         self._loop = loop