Streaming migration for 20.3 release (#1800)

* Compatibility and deprecations for Sanic 20.3 in preparation of the streaming branch.

* Add test for new API.

* isort tests

* More coverage

* json takes str, not bytes

Co-authored-by: L. Kärkkäinen <tronic@users.noreply.github.com>

tests/test_response.py

@@ -15,6 +15,7 @@ from aiofiles import os as async_os
 from sanic.response import (
     HTTPResponse,
     StreamingHTTPResponse,
+    empty,
     file,
     file_stream,
     json,
@@ -22,7 +23,6 @@ from sanic.response import (
     stream,
     text,
 )
-from sanic.response import empty
 from sanic.server import HttpProtocol
 from sanic.testing import HOST, PORT