[1.8.x] Sorted imports with isort; refs #23860.

Backport of 0ed7d15563 from master

tests/builtin_server/tests.py

@@ -1,12 +1,11 @@
 from __future__ import unicode_literals
 
-from io import BytesIO
 import sys
 import traceback
+from io import BytesIO
 from unittest import TestCase
 from wsgiref import simple_server
 
-
 # If data is too large, socket will choke, so write chunks no larger than 32MB
 # at a time. The rationale behind the 32MB can be found on Django's Trac:
 # https://code.djangoproject.com/ticket/5596#comment:4