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-unpack.py b/Doc/includes/email-unpack.py
index e0a7f01..c8cb0be 100644
--- a/Doc/includes/email-unpack.py
+++ b/Doc/includes/email-unpack.py
@@ -43,7 +43,7 @@
             if not ext:
                 # Use a generic bag-of-bits extension
                 ext = '.bin'
-            filename = 'part-%03d%s' % (counter, ext)
+            filename = f'part-{counter:03d}{ext}'
         counter += 1
         with open(os.path.join(args.directory, filename), 'wb') as fp:
             fp.write(part.get_payload(decode=True))