get tox to use lint extras

synapse/python_dependencies.py

@@ -104,7 +104,13 @@
     # hiredis is not a *strict* dependency, but it makes things much faster.
     # (if it is not installed, we fall back to slow code.)
     "redis": ["txredisapi>=1.4.7", "hiredis"],
-    "lint": ["isort==5.0.3", "black==19.10b0", "flake8-comprehensions", "flake8"],
+    # We pin black so that our tests don't start failing on new releases.
+    "lint": [
+        "isort<=5.0.3",
+        'black==19.10b0;python_version>"3.5"',
+        "flake8-comprehensions",
+        "flake8",
+    ],
 }
 
 ALL_OPTIONAL_REQUIREMENTS = set()  # type: Set[str]

tox.ini

@@ -118,20 +118,14 @@ commands =
     check-manifest
 
 [testenv:check_codestyle]
-skip_install = True
-deps =
-    flake8
-    flake8-comprehensions
-    # We pin so that our tests don't start failing on new releases of black.
-    black==19.10b0
+extras = lint
 commands =
     python -m black --check --diff .
     /bin/sh -c "flake8 synapse tests scripts scripts-dev contrib synctl {env:PEP8SUFFIX:}"
     {toxinidir}/scripts-dev/config-lint.sh
 
 [testenv:check_isort]
-skip_install = True
-deps = isort==5.0.3
+extras = lint
 commands = /bin/sh -c "isort -c --df --sp setup.cfg synapse tests scripts-dev scripts"
 
 [testenv:check-newsfragment]