Fix python 3.7 -> 3.7-dev and add flake8 testing

Also fixed flake8 errors.

testproj/snippets/admin.py

@@ -1,3 +1 @@
-from django.contrib import admin
-
 # Register your models here.

testproj/snippets/serializers.py

@@ -1,6 +1,5 @@
-from rest_framework.status import HTTP_400_BAD_REQUEST
-
 from rest_framework import serializers
+
 from snippets.models import Snippet, LANGUAGE_CHOICES, STYLE_CHOICES