deprecate the eventlet handler from the zookeeper driver

Change-Id: I050f520117687276cbd895079e292414b1d91619

releasenotes/notes/deprecate-kazoo-eventlet-handler-4f931cf058f38200.yaml

@@ -0,0 +1,11 @@
+---
+deprecations:
+  - |
+    Eventlet usages are deprecated and the removal of Eventlet from
+    OpenStack `is planned <https://governance.openstack.org/tc//goals/proposed/remove-eventlet.html>`_,
+    for this reason, using the ``eventlet`` handler from kazoo
+    available into  the zookeeper driver is now deprecated. The support of
+    Eventlet will be soon removed from tooz.
+
+    Please start considering removing your internal Eventlet usages and
+    start migrating your stack.

requirements.txt

@@ -10,3 +10,4 @@ tenacity>=5.0.0 # Apache-2.0
 futurist>=1.2.0 # Apache-2.0
 oslo.utils>=4.7.0 # Apache-2.0
 oslo.serialization>=1.10.0 # Apache-2.0
+debtcollector>=3.0.0 # Apache-2.0

tooz/drivers/zookeeper.py

@@ -13,6 +13,7 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+import debtcollector
 from kazoo import client
 from kazoo import exceptions
 from kazoo import security
@@ -494,6 +495,10 @@ class KazooDriver(coordination.CoordinationDriverCachedRunWatchers):
             'verify_certs': strutils.bool_from_string(verify_certs),
         }
         handler_kind = options.get('handler')
+        if handler_kind == "eventlet":
+            debtcollector.deprecate(
+                "Eventlet support is deprecated and will be removed. "
+                "Use threading handler instead.")
         if handler_kind:
             try:
                 handler_cls = self.HANDLERS[handler_kind]