Enforce repo-review rule

PP309: Filter warnings specified

`filterwarnings` must be set (probably to at least `["error"]`). Python
will hide important warnings otherwise, like deprecations.

pyproject.toml

@@ -69,6 +69,7 @@ repository = "https://github.com/ImagingDataCommons/highdicom.git"
 [tool.pytest.ini_options]
 minversion = "7"
 addopts = ["--doctest-modules", "-ra", "--strict-config", "--strict-markers"]
+filterwarnings = ["error"]
 testpaths = ["tests"]
 log_cli_level = "INFO"
 xfail_strict = true