Update testing libs

setup.py

@@ -39,10 +39,11 @@ setup(
     ],
     extras_require={
         "test": [
-            "tox==3.14.3",
-            "mysqlclient==1.4.6",
-            "psycopg2-binary==2.8.4",
-            "django_add_default_value==0.3.2",
+            "tox>=3.15.2",
+            "mysqlclient>=1.4.6",
+            "psycopg2-binary>=2.8.5",
+            "django_add_default_value>=0.4.0",
+            'mock>=3.0.5;python_version<"3.3"',
         ]
     },
     keywords="django migration lint linter database backward compatibility",

tox.ini

@@ -15,9 +15,6 @@ deps =
     django21: django>=2.1,<2.2
     django22: django>=2.2,<2.3
     django30: django>=3.0,<3.1
-    appdirs==1.4.3
-    six==1.14.0
-    py27: mock==2.0.0
 
 [testenv:lint]
 basepython = python3.6