CMFActivity.ActivityTool: Use context managers for locking.

Not all lock uses in this code fit context managers, but use them where it
does.

product/CMFActivity/ActivityTool.py

@@ -1300,14 +1300,12 @@ class ActivityTool (BaseTool):
 
       # return if the number of threads is too high
       # else, increase the number of active_threads and continue
-      tic_lock.acquire()
+      with tic_lock:
         too_many_threads = (active_threads >= max_active_threads)
         if not too_many_threads or force:
           active_threads += 1
         else:
-        tic_lock.release()
           raise RuntimeError, 'Too many threads'
-      tic_lock.release()
 
       inner_self = aq_inner(self)
 
@@ -1338,9 +1336,8 @@ class ActivityTool (BaseTool):
             is_running_lock.release()
       finally:
         # decrease the number of active_threads
-        tic_lock.acquire()
+        with tic_lock:
           active_threads -= 1
-        tic_lock.release()
 
     def hasActivity(self, *args, **kw):
       # Check in each queue if the object has deferred tasks