CI: treat warnings as errors in tests

pytest.ini

@@ -13,13 +13,25 @@ norecursedirs =
   build
 
 python_files = test_*.py
-addopts = --doctest-glob="*.rst" --xdoctest --mpl --mpl-baseline-path=fastf1/tests/mpl-baseline --mpl-results-path=mpl-results
+addopts =
+  --doctest-glob="*.rst"
+  --xdoctest
+  --mpl
+  --mpl-baseline-path=fastf1/tests/mpl-baseline
+  --mpl-results-path=mpl-results
 
 
 filterwarnings =
+  # turn all warnings into errors
+  # (the last matching action is performed, therefore error needs to be first
+  error
+  #
   # fastf1 internal, only relevant for users
   # (10/2022)
   ignore:.*to_datetime.*will return NaT instead of raising an exception.*:FutureWarning
+  # (10/2023)
+  ignore:.*`utils\.delta_time` is considered deprecated.*:FutureWarning
+  #
   # external, verified as not relevant
   # (10/2022)
   ignore:.*df.iloc.*will attempt to set the values inplace.*:FutureWarning