Add tox and CI job to check all pre-commit hooks

This will only check the latest commit, not all the commits in the PR,
but that's better than nothing.

Author: pganssle

.github/workflows/tests.yml

@@ -31,7 +31,7 @@ jobs:
     runs-on: "ubuntu-latest"
     strategy:
       matrix:
-        toxenv: ["build", "pytype", "docs"]
+        toxenv: ["build", "precommit", "pytype", "docs"]
     env:
       TOXENV: ${{ matrix.toxenv }}
 

tox.ini

@@ -12,6 +12,14 @@ deps =
 commands =
     pytest {toxinidir} {posargs}
 
+[testenv:precommit]
+description = Run the pre-commit hooks on all files
+deps =
+    pre-commit
+commands =
+    pre-commit install -f --install-hooks
+    pre-commit run --all-files
+
 [testenv:update]
 description = Update the tzdata contents
 skip_install = True
@@ -30,7 +38,6 @@ deps =
 commands =
     python bump_version.py {posargs}
 
-
 [testenv:pytype]
 description = Run typechecking
 basepython = python3.7