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/ftplib.py b/Lib/ftplib.py
index 4d92b86..1d3bd9e 100644
--- a/Lib/ftplib.py
+++ b/Lib/ftplib.py
@@ -287,7 +287,7 @@
         return self.voidcmd(cmd)
 
     def sendeprt(self, host, port):
-        '''Send a EPRT command with the current host and the given port number.'''
+        '''Send an EPRT command with the current host and the given port number.'''
         af = 0
         if self.af == socket.AF_INET:
             af = 1
@@ -852,7 +852,7 @@
 
 
 def parse229(resp, peer):
-    '''Parse the '229' response for a EPSV request.
+    '''Parse the '229' response for an EPSV request.
     Raises error_proto if it does not contain '(|||port|)'
     Return ('host.addr.as.numbers', port#) tuple.'''