Handle missing 'tests_require' option in newer setuptools (#59)

This option was outright removed from newer versions of setuptools and
is no longer visible at all. The existing behavior always raises an
exception even if the unsupported option isn't used in the package.

colcon_python_setup_py/package_augmentation/python_setup_py.py

@@ -44,7 +44,7 @@ def augment_package(  # noqa: D102
         ]:
             desc.dependencies[dependency_type] = {
                 create_dependency_descriptor(d)
-                for d in config[option_name] or ()}
+                for d in config.get(option_name) or ()}
 
         def getter(env):
             nonlocal setup_py