Add pytest-style linting (#159)

This does mean we need to tell pyright that the init_cache auto-use
fixture is still being used even though it now has a leading underscore.

tests/pyproject.toml

@@ -1,5 +1,8 @@
 [tool.ruff]
 extend = "../pyproject.toml"
+extend-select = [
+    "PT", # flake8-pytest-style
+]
 ignore = ["S101"]
 
 [tool.ruff.isort]