MAINT: Resolve DeprecationWarnings

Resolves the following two DeprecationWarnings from traitlets 4.1:

    DeprecationWarning: Traits should be given as instances, not types
    (for example, `Int()`, not `Int`). Passing types is deprecated in
    traitlets 4.1.

    DeprecationWarning: metadata {'example': 97} was set from the
    constructor. With traitlets 4.1, metadata should be set using the
    .tag() method, e.g., Int().tag(key1='value1', key2='value2')

straitlets/builtin_models.py

@@ -127,7 +127,7 @@ def _password_requires_user(self, proposal):
         return new
 
     hosts = List(
-        trait=Unicode,
+        trait=Unicode(),
         minlen=1,
         help=(
             "List of hosts in the replicaset.  "

straitlets/tests/test_examples.py

@@ -152,14 +152,14 @@ def test_nested_example():
 
     class C(Serializable):
         point = Instance(Point)
-        unicode_ = Unicode(example='foo')
+        unicode_ = Unicode().tag(example='foo')
 
     class B(Serializable):
-        value = Integer(example=ord('b'))
+        value = Integer().tag(example=ord('b'))
         next_ = Instance(C)
 
     class A(Serializable):
-        value = Integer(example=ord('a'))
+        value = Integer().tag(example=ord('a'))
         next_ = Instance(B)
 
     expected = A(