patchbomb: use email.mime.base instead of email.MIMEBase

The later was removed on Python 3.

On python 2:

>>> email.MIMEBase.MIMEBase is email.mime.base.MIMEBase
True

Differential Revision: https://phab.mercurial-scm.org/D3836

hgext/patchbomb.py

@@ -75,6 +75,7 b' from __future__ import absolute_import'
 
 import email as emailmod
 import email.generator as emailgen
+import email.mime.base as emimebase
 import email.mime.multipart as emimemultipart
 import email.utils as eutil
 import errno
@@ -369,7 +370,7 b' def _getbundlemsgs(repo, sender, bundle,'
     msg = emimemultipart.MIMEMultipart()
     if body:
         msg.attach(mail.mimeencode(ui, body, _charsets, opts.get(r'test')))
-    datapart = emailmod.MIMEBase.MIMEBase('application', 'x-mercurial-bundle')
+    datapart = emimebase.MIMEBase('application', 'x-mercurial-bundle')
     datapart.set_payload(bundle)
     bundlename = '%s.hg' % opts.get(r'bundlename', 'bundle')
     datapart.add_header('Content-Disposition', 'attachment',