Why not support multi-hosts in etcd v3 client

In etcd v2 python client, It is allowed to input multi-hosts to improve the high availability, But in etcd v3 python client, only one host is allowed , anyone knows why?

**V3:**
```
class Etcd3Client(object):
def __init__(self, host='localhost', port=2379,
ca_cert=None, cert_key=None, cert_cert=None, timeout=None,
user=None, password=None, grpc_options=None):

self._url = '{host}:{port}'.format(host=host, port=port)
self.metadata = None
```

**v2:**

```
class Client(object):

"""
Client for etcd, the distributed log service using raft.
"""

_MGET = 'GET'
_MPUT = 'PUT'
_MPOST = 'POST'
_MDELETE = 'DELETE'
_comparison_conditions = set(('prevValue', 'prevIndex', 'prevExist', 'refresh'))
_read_options = set(('recursive', 'wait', 'waitIndex', 'sorted', 'quorum'))
_del_conditions = set(('prevValue', 'prevIndex'))

http = None

def __init__(
self,
host='127.0.0.1',
port=4001,
srv_domain=None,
version_prefix='/v2',
read_timeout=60,
allow_redirect=True,
protocol='http',
cert=None,
ca_cert=None,
username=None,
password=None,
allow_reconnect=False,
use_proxies=False,
expected_cluster_id=None,
per_host_pool_size=10,
lock_prefix="/_locks"
):
"""
Initialize the client.

Args:
host (mixed):
If a string, IP to connect to.
**If a tuple ((host, port), (host, port), ...)**

port (int): Port used to connect to etcd.
```

Start with the Etcd3Client.__init__ signature and _url construction shown in the issue, then compare how the v2 Client accepts a tuple of hosts. Define how multiple endpoints should be selected for high availability and verify that the client can connect through them; the payload names no tests or additional files.

Written by the indexing model from the issue text.