Workaround tornado.testing defect

This is fixed in the mainline but I'm going to downgrade pytest for the
time being.

tornadoweb/tornado#3382

pyproject.toml

@@ -41,7 +41,7 @@ dependencies = [
     "flake8-fixme",
     "flake8-print",
     "flake8-pyproject",
-    "pytest",
+    "pytest>8.1,<8.2",  # https://github.com/tornadoweb/tornado/pull/3382
     "python-docs-theme",
     "readme-renderer",
     "sphinx",