Use modern "with open() as file"

Saves a LOC and guarantees that the file will be closed

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

offlineimap/folder/Base.py

@@ -154,9 +154,8 @@ class BaseFolder(object):
         newval = self.get_uidvalidity()
         uidfilename = self._getuidfilename()
 
-        file = open(uidfilename + ".tmp", "wt")
+        with open(uidfilename + ".tmp", "wt") as file:
             file.write("%d\n" % newval)
-        file.close()
         os.rename(uidfilename + ".tmp", uidfilename)
         self._base_saved_uidvalidity = newval