update test requirements

Author: philall

requirements-test.txt

@@ -1,6 +1,6 @@
 Django>=1.6
 djangorestframework>=2.4.3
-pytest-django==2.6
-pytest==2.5.2
-pytest-cov==1.6
-flake8==2.2.2
+pytest-django==3.1.2
+pytest==3.1.3
+pytest-cov==2.5.1
+flake8==3.3.0