BOM-617 Fix test_static_replace test failures.

Update the tests around static_file code to use byte streams instead of
string streams for generating static content.  This is a fix to get
these tests passing on python 3.

common/djangoapps/static_replace/test/test_static_replace.py

@@ -3,7 +3,7 @@
 from __future__ import absolute_import, print_function
 
 import re
-from six import StringIO
+from six import BytesIO
 from six.moves.urllib.parse import parse_qsl, urlparse, urlunparse
 
 import ddt
@@ -331,7 +331,7 @@ class CanonicalContentTest(SharedModuleStoreTestCase):
             StaticContent: the StaticContent object for the created image
         """
         new_image = Image.new('RGB', dimensions, color)
-        new_buf = StringIO()
+        new_buf = BytesIO()
         new_image.save(new_buf, format='png')
         new_buf.seek(0)
         new_name = name.format(prefix)
@@ -355,7 +355,7 @@ class CanonicalContentTest(SharedModuleStoreTestCase):
             StaticContent: the StaticContent object for the created content
 
         """
-        new_buf = StringIO('testingggggggggggg')
+        new_buf = BytesIO(b'testingggggggggggg')
         new_name = name.format(prefix)
         new_key = StaticContent.compute_location(cls.courses[prefix].id, new_name)
         new_content = StaticContent(new_key, new_name, 'application/octet-stream', new_buf.getvalue(), locked=locked)