Deprecate the etcd3 driver

The etcd3 driver depends on the python-etcd3 [1]
library that is currently stale and no has no
active maintenance [2] with a non-working CI
that blocks all PRs.

It's also currently blocking Tooz from supporting
newer versions of tenacity [3] that a lot of
projects is now pinning because newer version of
tenacity dropped old legacy compatibility code
that has been in there for years.

We currently have etcd3gw as an alternative driver
where the library the driver depends on is a
deliverable that we maintain.

[1] https://github.com/kragniz/python-etcd3
[2] https://github.com/kragniz/python-etcd3/issues/1719
[3] https://review.opendev.org/c/openstack/tooz/+/829412

Change-Id: Iff0cd3b51cfc45ecbddc831c55267e80e9c79cac

.zuul.yaml

@@ -10,8 +10,6 @@
       jobs:
         - tooz-tox-py38-etcd
         - tooz-tox-py36-etcd
-        - tooz-tox-py38-etcd3
-        - tooz-tox-py36-etcd3
         - tooz-tox-py38-etcd3gw
         - tooz-tox-py36-etcd3gw
         - tooz-tox-py38-zookeeper
@@ -32,8 +30,6 @@
       jobs:
         - tooz-tox-py38-etcd
         - tooz-tox-py36-etcd
-        - tooz-tox-py38-etcd3
-        - tooz-tox-py36-etcd3
         - tooz-tox-py38-etcd3gw
         - tooz-tox-py36-etcd3gw
         - tooz-tox-py38-zookeeper
@@ -67,14 +63,6 @@
     vars:
       tox_envlist: py38-etcd
 
-- job:
-    name: tooz-tox-py38-etcd3
-    parent: openstack-tox-py38
-    description: |
-      Run tests using ``py38-etcd3`` environment.
-    vars:
-      tox_envlist: py38-etcd3
-
 - job:
     name: tooz-tox-py38-etcd3gw
     parent: openstack-tox-py38
@@ -151,14 +139,6 @@
     vars:
       tox_envlist: py36-etcd
 
-- job:
-    name: tooz-tox-py36-etcd3
-    parent: openstack-tox-py36
-    description: |
-      Run tests using ``py36-etcd3`` environment.
-    vars:
-      tox_envlist: py36-etcd3
-
 - job:
     name: tooz-tox-py36-etcd3gw
     parent: openstack-tox-py36

releasenotes/notes/deprecate-etcd3-driver-551608037d79222b.yaml

@@ -0,0 +1,5 @@
+---
+deprecations:
+  - |
+    The etcd3 driver is deprecated, it will be removed in a future
+    release. No testing is performed on this driver.

tooz/drivers/etcd3.py

@@ -25,6 +25,8 @@ from tooz import coordination
 from tooz import locking
 from tooz import utils
 
+import warnings
+
 
 @contextlib.contextmanager
 def _failure_translator():
@@ -143,6 +145,10 @@ class Etcd3Driver(coordination.CoordinationDriverCachedRunWatchers,
 
     def __init__(self, member_id, parsed_url, options):
         super(Etcd3Driver, self).__init__(member_id, parsed_url, options)
+        warnings.warn(
+            "The etcd3 tooz driver is deprecated, it will be removed in"
+            "a future release."
+        )
         host = parsed_url.hostname or self.DEFAULT_HOST
         port = parsed_url.port or self.DEFAULT_PORT
         options = utils.collapse(options)