Merge the pytest configuration (#446)

The addition of the pytest.ini is eclipsing the pytest configuration
that was already present in the setup.cfg.

There can be only one.

This moves the configuration from the pytest.ini into the setup.cfg and
re-adjusts the warnings filter to get things passing on all of Ubuntu,
macOS, and Windows.

setup.cfg

@@ -65,9 +65,16 @@ exclude =
 [tool:pytest]
 filterwarnings =
     error
-    ignore:the imp module is deprecated in favour of importlib.*:DeprecationWarning
-    ignore:the imp module is deprecated in favour of importlib.*:PendingDeprecationWarning
+    # Suppress deprecation warnings in other packages
+    ignore:lib2to3 package is deprecated::scspell
+    ignore:SelectableGroups dict interface is deprecated::flake8
+    ignore:The loop argument is deprecated::asyncio
+    ignore:Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated::pydocstyle
+    ignore:Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated::pyreadline
+    # Suppress resource warnings pending investigation
+    always::ResourceWarning
 junit_suite_name = colcon-core
+python_classes = !TestFailure
 
 [options.entry_points]
 colcon_core.argument_parser =