mail: use a context manager when writing to mbox

Differential Revision: https://phab.mercurial-scm.org/D10484

mercurial/mail.py

@@ -208,7 +208,7 b' def _sendmail(ui, sender, recipients, ms'
 
 def _mbox(mbox, sender, recipients, msg):
     '''write mails to mbox'''
-    fp = open(mbox, b'ab+')
+    with open(mbox, b'ab+') as fp:
     # Should be time.asctime(), but Windows prints 2-characters day
     # of month instead of one. Make them print the same thing.
     date = time.strftime('%a %b %d %H:%M:%S %Y', time.localtime())
@@ -218,7 +218,6 b' def _mbox(mbox, sender, recipients, msg)'
     )
     fp.write(msg)
     fp.write(b'\n\n')
-    fp.close()
 
 
 def connect(ui, mbox=None):