Add tox and ruff to dev dependencies and update testing instructions

TESTING.rst

@@ -1,9 +1,9 @@
 Testing
 =======
 
-First, install tox into your virtualenv::
+First, install dev dependencies::
 
-    pip install --upgrade tox
+    poetry install --with dev
 
 To run all tests, you'll need to install multiple Python interpreters. On a
 modern Ubuntu distribution you can use ``add-apt-repository
@@ -21,3 +21,11 @@ Here's the OSX Homebrew command:
 
 Finally, just run ``tox``!
 If you want, you can test against a specific version like this: ``tox -e py312-pil``
+
+
+Linting
+-------
+
+Run `ruff` to check linting::
+
+    ruff check

pyproject.toml

@@ -52,6 +52,8 @@ png = ["pypng"]
 [tool.poetry.group.dev.dependencies]
 pytest = {version = "*"}
 pytest-cov = {version = "*"}
+tox = {version = "*"}
+ruff = {version = "*"}
 docutils = "^0.21.2"
 zest-releaser = {extras = ["recommended"], version = "^9.2.0"}