threadutil.py changed not var in

This patch changes:

if not var1 in var2

with

if var1 not in var2

offlineimap/threadutil.py

@@ -199,7 +199,7 @@ def initInstanceLimit(limitNamespace, instancemax):
 
     global limitedNamespaces
 
-    if not limitNamespace in limitedNamespaces:
+    if limitNamespace not in limitedNamespaces:
         limitedNamespaces[limitNamespace] = BoundedSemaphore(instancemax)