Update testing dependencies (mysqlclient, do not let Django 2.2 overlap to 3.0 under dev, and update black)

tox.ini

@@ -12,15 +12,15 @@ deps =
     django111: django>=1.11,<2.0
     django20: django>=2.0,<2.1
     django21: django>=2.1,<2.2
-    django22: django>=2.2
+    django22: django>=2.2,<2.3
     appdirs==1.4.3
     py27: mock==2.0.0
 
 [testenv:lint]
 basepython = python3.6
 deps =
-    flake8==3.7.8
-    black==19.3b0
+    flake8==3.7.9
+    black==19.10b0
 commands =
     flake8 --max-line-length=88 django_migration_linter
     black --check django_migration_linter/ tests/ manage.py setup.py