prevent mutable in parameter default

https://app.codacy.com/app/revol/etcd3-py/pullRequest?prid=3161986

Author: dariko

etcd3/baseclient.py

@@ -59,7 +59,9 @@ def __init__(self, host=None, port=None, endpoints=None, protocol='http', cert=(
                  verify=None, timeout=None, headers=None, user_agent=None, pool_size=30,
                  username=None, password=None, token=None,
                  server_version=DEFAULT_VERSION, cluster_version=DEFAULT_VERSION,
-                 failover_whitelist=["range", "watch", "status"]):
+                 failover_whitelist=None):
+        if failover_whitelist is None:
+            failover_whitelist = ["range", "watch", "status"]
         if host is not None:
             self.endpoints = ([EtcdEndpoint(host, port)])
         else: