Fixed #18457 -- Fixed encoding error in yaml deserializer

Thanks jpaugh64 for the report.

django/core/serializers/pyyaml.py

@@ -12,6 +12,8 @@
 from django.core.serializers.base import DeserializationError
 from django.core.serializers.python import Serializer as PythonSerializer
 from django.core.serializers.python import Deserializer as PythonDeserializer
+from django.utils.encoding import smart_str
+
 
 class DjangoSafeDumper(yaml.SafeDumper):
     def represent_decimal(self, data):
@@ -48,8 +50,10 @@ def Deserializer(stream_or_string, **options):
     """
     Deserialize a stream or string of YAML data.
     """
-    if isinstance(stream_or_string, basestring):
+    if isinstance(stream_or_string, bytes):
         stream = BytesIO(stream_or_string)
+    if isinstance(stream_or_string, unicode):
+        stream = BytesIO(smart_str(stream_or_string))
     else:
         stream = stream_or_string
     try: