run linting/typechecking only once

setup.cfg

@@ -30,19 +30,24 @@ console_scripts =
     basicpy = basicpy:main
 
 [tox:tox]
-envlist = py38, py39
+envlist = lint, py38, py39
 isolated_build = True
 
-[testenv]
+[testenv:lint]
+skip_install = True
 deps =
     black
     flake8
     mypy
-    pytest
 commands =
     black --check --diff src tests
     flake8 src tests
     mypy --disallow-untyped-defs src tests
+
+[testenv]
+deps =
+    pytest
+commands =
     pytest tests
 
 [flake8]