tox: remove isort

The role of the `isort` check is now performed by `ruff` instead of
`isort`.

Makefile

@@ -248,7 +248,7 @@ test-all:
 
 .PHONY: lint
 lint:
-	tox run-parallel -e ruff,pylint,autopep8,isort,mypy,mypy-strict
+	tox run-parallel -e ruff,pylint,autopep8,mypy,mypy-strict
 
 #
 # Building packages

tox.ini

@@ -6,7 +6,7 @@ env_list =
 
 labels =
     test = py{36,37,38,39,310,311}
-    lint = ruff, isort, autopep8, pylint
+    lint = ruff, autopep8, pylint
     type = mypy,mypy-strict
 
 [testenv]
@@ -45,13 +45,6 @@ deps =
 commands =
     bash -c 'python -m ruff check {env:LINTABLES}'
 
-[testenv:isort]
-deps =
-    isort==5.12.0
-
-commands =
-    bash -c 'python -m isort --check --diff {env:LINTABLES}'
-
 [testenv:autopep8]
 deps =
     autopep8==2.0.2