ci: streamline tox env dependencies

Also makes it easier to install locally

setup.cfg

@@ -46,7 +46,10 @@ install_requires =
 
 [options.extras_require]
 testing =
-    tox >= 3, <4
+    pytest-django >= 3, <4
+codestyle =
+    flake8 >= 3, <4
+    black==20.8b1
 
 [options.entry_points]
 console_scripts =
@@ -69,7 +72,7 @@ envlist =
 [testenv]
 # Run test suite
 deps =
-    pytest-django >=3, <4
+    .[testing]
     dj22: Django>=2.2, <3
     dj30: Django>=3.0, <3.1
     dj31: Django>=3.1, <3.2
@@ -96,26 +99,23 @@ commands =
     twine check "{envtmpdir}/dist/*"
     check-manifest --ignore-bad-ideas '*.mo' {toxinidir}
 
-[testenv:black]
-# Lint with black
-skip_install = true
-deps =
-    black==20.8b1
-commands = black . --check --exclude "(\.tox|\.git|build|dist|migrations)"
-
 [testenv:format]
 # This env is not run by default. It's provided here for you
 # to easily autoformat code by running `tox -e format`
 skip_install = true
-deps =
-    black==20.8b1
-commands = black . --exclude "(\.tox|\.git|build|dist|migrations)"
+deps = .[codestyle]
+commands = black .
+
+[testenv:black]
+# Lint with black
+skip_install = true
+deps = .[codestyle]
+commands = black . --check
 
 [testenv:flake8]
 # Lint with flake8
 skip_install = true
-deps =
-    flake8 >= 3, <4
+deps = .[codestyle]
 commands = flake8 .
 
 [flake8]