Upgrade pytest configuration (#723)

Use the new proper TOML support [added in pytest
9](https://docs.pytest.org/en/stable/changelog.html#pytest-9-0-0-2025-11-05).

Author: adamchainz

pyproject.toml

@@ -50,7 +50,7 @@ test = [
   "faker",
   "model-bakery>=1.13",
   "numpy",
-  "pytest",
+  "pytest>=9",
   "pytest-xdist",
 ]
 
@@ -107,11 +107,7 @@ warn_unreachable = true
 overrides = [ { module = "tests.*", allow_untyped_defs = true } ]
 
 [tool.pytest]
-ini_options.addopts = """\
-    --strict-config
-    --strict-markers
-    """
-ini_options.xfail_strict = true
+strict = true
 
 [tool.coverage]
 run.branch = true