Bring back tox -e linting

It just runs pre-commit, and CI doesn't use i,t but I still find it
useful for local development.

tox.ini

@@ -30,6 +30,14 @@ deps = pytest
 commands =
   pytest {posargs}
 
+[testenv:linting]
+skip_install = True
+usedevelop = True
+passenv = PRE_COMMIT_HOME
+deps =
+  pre-commit
+commands = pre-commit run --all-files --show-diff-on-failure
+
 [testenv:release]
 changedir=
 decription = do a release, required posarg of the version number