Improve type checking (backport #679)

[mergify]

This is still a work in progress, but following @clalancette 's comments on #676 , I made this PR to:

• Introduce the mypy type checker, and check how long it takes to run (On my machine a couple of seconds)
• Show some examples of fixes that are necessary for some files to pass

Note that we will probably need to tweak the mypy configuration a little bit, since currently non-typed imports are treated as errors.

---

This is an automatic backport of pull request #679 done by Mergify.

[mergify]

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Changes to be committed:
	modified:   launch/doc/source/conf.py
	modified:   launch/examples/disable_emulate_tty_counters.py
	modified:   launch/examples/launch_counters.py
	modified:   launch/launch/__init__.py
	modified:   launch/launch/action.py
	modified:   launch/launch/actions/declare_launch_argument.py
	modified:   launch/launch/actions/execute_local.py
	modified:   launch/launch/actions/execute_process.py
	modified:   launch/launch/actions/opaque_coroutine.py
	modified:   launch/launch/actions/register_event_handler.py
	modified:   launch/launch/actions/timer_action.py
	modified:   launch/launch/conditions/launch_configuration_equals.py
	modified:   launch/launch/descriptions/executable.py
	modified:   launch/launch/event_handler.py
	modified:   launch/launch/event_handlers/on_action_event_base.py
	modified:   launch/launch/event_handlers/on_execution_complete.py
	modified:   launch/launch/event_handlers/on_process_exit.py
	modified:   launch/launch/event_handlers/on_process_io.py
	modified:   launch/launch/event_handlers/on_process_start.py
	modified:   launch/launch/event_handlers/on_shutdown.py
	modified:   launch/launch/events/process/process_targeted_event.py
	modified:   launch/launch/events/process/shutdown_process.py
	modified:   launch/launch/events/process/signal_process.py
	modified:   launch/launch/frontend/entity.py
	modified:   launch/launch/frontend/expose.py
	modified:   launch/launch/frontend/parser.py
	modified:   launch/launch/launch_context.py
	modified:   launch/launch/launch_description_source.py
	modified:   launch/launch/launch_description_sources/any_launch_file_utilities.py
	modified:   launch/launch/launch_description_sources/frontend_launch_file_utilities.py
	modified:   launch/launch/launch_description_sources/python_launch_file_utilities.py
	modified:   launch/launch/launch_introspector.py
	modified:   launch/launch/launch_service.py
	modified:   launch/launch/logging/handlers.py
	modified:   launch/launch/some_actions_type.py
	new file:   launch/launch/some_entities_type.py
	modified:   launch/launch/substitutions/anon_name.py
	modified:   launch/launch/substitutions/boolean_substitution.py
	modified:   launch/launch/substitutions/command.py
	modified:   launch/launch/substitutions/environment_variable.py
	modified:   launch/launch/substitutions/equals_substitution.py
	modified:   launch/launch/substitutions/find_executable.py
	modified:   launch/launch/substitutions/launch_configuration.py
	modified:   launch/launch/substitutions/path_join_substitution.py
	modified:   launch/launch/utilities/__init__.py
	new file:   launch/launch/utilities/normalize_to_list_of_entities_impl.py
	modified:   launch/launch/utilities/signal_management.py
	modified:   launch/launch/utilities/type_utils.py
	modified:   launch/package.xml
	new file:   launch/setup.cfg
	modified:   launch/test/launch/frontend/test_substitutions.py
	modified:   launch/test/launch/test_event_handler.py
	modified:   launch/test/launch/utilities/test_signal_management.py
	modified:   launch/test/launch/utilities/test_type_utils.py
	new file:   launch/test/test_mypy.py
	modified:   launch_testing/launch_testing/actions/test.py
	modified:   launch_testing/launch_testing/event_handlers/stdout_ready_listener.py

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both modified:   launch/launch/logging/__init__.py
	deleted by us:   launch/launch/substitutions/launch_log_dir.py
	both modified:   launch/launch/substitutions/python_expression.py
	both modified:   launch/launch/substitutions/this_launch_file.py
	both modified:   launch/launch/substitutions/this_launch_file_dir.py

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally