Replace has_key method to "key in dict"

There is no such method in Python3 any more.

Signed-off-by: Łukasz Żarnowiecki <dolohow@outlook.com>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/folder/Maildir.py

@@ -47,7 +47,7 @@ def _gettimeseq(date=None):
     try:
         if date is None:
             date = int(time.time())
-        if timehash.has_key(date):
+        if date in timehash:
             timehash[date] += 1
         else:
             timehash[date] = 0