Add pre-commit.

Makefile

@@ -0,0 +1,13 @@
+.PHONY: pre-commit clean test
+
+pre-commit:
+	@tox -e pre-commit -- install -f --install-hooks
+
+test: pre-commit
+	@tox -e pre-commit -- run --all-files
+	@tox
+
+clean:
+	@rm -fr .tox
+	@find . -name '*.py[co]' -delete
+	@find . -name '__pycache__' -delete