Suppress flake8 A005 in existing colcon API (#636)

Rather than suppress A005 completely, we can ignore it in our existing
API to prevent new A005 violations from appearing.

setup.cfg

@@ -156,6 +156,10 @@ colcon_core.task.python.template = *.em
 
 [flake8]
 import-order-style = google
+per-file-ignores =
+    colcon_core/distutils/__init__.py:A005
+    colcon_core/logging.py:A005
+    colcon_core/subprocess.py:A005
 
 [coverage:run]
 source = colcon_core