bpo-38351: Modernize email examples from %-formatting to f-strings (GH-17162)

(cherry picked from commit e8acc865a3f112b98417f676c897ca6ec2dac2c7)

Co-authored-by: Andrey Doroschenko <dorosch.github.io@yandex.ru>
diff --git a/Doc/includes/email-dir.py b/Doc/includes/email-dir.py
index 0dcfbfb..2fc1570 100644
--- a/Doc/includes/email-dir.py
+++ b/Doc/includes/email-dir.py
@@ -41,7 +41,7 @@
         directory = '.'
     # Create the message
     msg = EmailMessage()
-    msg['Subject'] = 'Contents of directory %s' % os.path.abspath(directory)
+    msg['Subject'] = f'Contents of directory {os.path.abspath(directory)}'
     msg['To'] = ', '.join(args.recipients)
     msg['From'] = args.sender
     msg.preamble = 'You will not see this in a MIME-aware mail reader.\n'