refactor: remove unecessary import guard for gevent.lock

The locks have been moved from `gevent.coros` to `gevent.lock`
since version 1.2 ([see changelog](https://www.gevent.org/changelog_1_2.html#a1-oct-27-2016))

kazoo/handlers/gevent.py

@@ -14,10 +14,7 @@
 
 from kazoo.handlers.utils import selector_select
 
-try:
-    from gevent.lock import Semaphore, RLock
-except ImportError:
-    from gevent.coros import Semaphore, RLock
+from gevent.lock import Semaphore, RLock
 
 from kazoo.handlers import utils