Upgrade pytest due to attrs incompatibility

pytest-dev/pytest#5903

Author: Joshua Massover

tox.ini

@@ -5,7 +5,7 @@ envlist = py26, py27, py33, py34, py35, py36, py37, pypy, pypy3, pycodestyle, co
 commands = pytest
 setenv = DJANGO_SETTINGS_MODULE=jsonrpc.tests.test_backend_django.settings
 deps =
-    pytest==4.0.2
+    pytest==4.3.0
     Django==1.11
     Flask==0.12.2