Use flake8 plugins for black and isort.

setup.cfg

@@ -65,4 +65,6 @@ test = coverage; fixtures; testtools; testresources; hypothesis; openapi_spec_va
 # Reference:
 #   https://flake8.pycqa.org/en/latest/user/error-codes.html
 #   https://pycodestyle.pycqa.org/en/latest/intro.html#error-codes
-select = F, W191
+#   https://pypi.org/project/flake8-isort/#error-codes
+#   https://pypi.org/project/flake8-black/#flake8-validation-codes
+select = F, W191, I, BLK

tests.nix

@@ -42,6 +42,8 @@ let
         isort == 5.10.1
         black == 21.12b0
         flake8 == 4.0.1
+        flake8-isort
+        flake8-black
       '';
     };
 
@@ -53,8 +55,6 @@ let
       mkdir -p $out
 
       pushd ${zkapauthorizer.src}
-      ${lint-python}/bin/black --check src
-      ${lint-python}/bin/isort --check src
       ${lint-python}/bin/flake8 src
       popd