Change email package module names to conform v4 (issue2550875)

Update the mailer roundup module to use email package names introduced
in v4 (Python v2.5). If nothing else this will provide forward support
for Python v3+. Thanks to Anthony Pankov for the patch.

Author: jerrykan

doc/acknowledgements.txt

@@ -109,6 +109,7 @@ Bryce L Nordgren,
 Patrick Ohly,
 "om",
 Luke Opperman,
+Anthony Pankov,
 Eddie Parker,
 Will Partain,
 Timo Paulssen,

roundup/mailer.py

@@ -9,12 +9,12 @@
 from roundup import __version__
 from roundup.date import get_timezone, Date
 
-from email.Utils import formatdate, formataddr, specialsre, escapesre
-from email.Message import Message
-from email.Header import Header
-from email.MIMEBase import MIMEBase
-from email.MIMEText import MIMEText
-from email.MIMEMultipart import MIMEMultipart
+from email.utils import formatdate, formataddr, specialsre, escapesre
+from email.message import Message
+from email.header import Header
+from email.mime.base import MIMEBase
+from email.mime.text import MIMEText
+from email.mime.multipart import MIMEMultipart
 
 try:
     import pyme, pyme.core