Be test-runner friendly by only decorating stdout/stderr if status line enabled.

[mikepurvis]

I have an in-progress colcon verb extension and I have a test for it which calls it via colcon_core.commands.verb_main. This works on its own and when run via nose, but fails with with an error when run via coverage, I guess because sys.stdout and sys.stderr are being overwritten in that environment to be StringIO objects which don't have the buffer attribute:

colcon.colcon_core.plugin_system: ERROR: Exception instantiating extension 'colcon_core.event_handler.status': '_io.StringIO' object has no attribute 'buffer'
Traceback (most recent call last):
  File "/home/administrator/colcon-env/lib/python3.8/site-packages/colcon_core/plugin_system.py", line 60, in _instantiate_extension
    extension_instance = extension_class()
  File "/home/administrator/colcon-env/lib/python3.8/site-packages/colcon_notification/event_handler/status.py", line 63, in __init__
    sys.stdout.buffer.write)
AttributeError: '_io.StringIO' object has no attribute 'buffer

It ends up being a bit of a cascade, because colcon actually traps this exception and carries on, with colcon-notification in a half-initialized state, so later on there are other failures from the rest of the constructor having not executed:

colcon.colcon_core.event_reactor: ERROR: Exception in event handler extension 'summary': 'StatusEventHandler' object has no attribute '_last_status_line_length'
Traceback (most recent call last):
  File "/home/administrator/colcon-env/lib/python3.8/site-packages/colcon_core/event_reactor.py", line 78, in _notify_observers
    retval = observer(event)
  File "/home/administrator/colcon-env/lib/python3.8/site-packages/colcon_output/event_handler/summary.py", line 65, in __call__
    self._print_summary()
  File "/home/administrator/colcon-env/lib/python3.8/site-packages/colcon_output/event_handler/summary.py", line 69, in _print_summary
    print()
  File "/home/administrator/colcon-env/lib/python3.8/site-packages/colcon_notification/event_handler/status.py", line 75, in wrapped_func
    self._clear_last_status_line()
  File "/home/administrator/colcon-env/lib/python3.8/site-packages/colcon_notification/event_handler/status.py", line 80, in _clear_last_status_line
    if self._last_status_line_length is not None:
AttributeError: 'StatusEventHandler' object has no attribute '_last_status_line_length

This change addresses the issue by not trying to decorate in the non-tty case.

[mikepurvis]

Looks like the test failure is just an internal flake8 warning and is patched as of a week ago, see: python/importlib_metadata#298 (comment) but awaiting a flake8 release, as the current one is a month old.

EDIT: Oh wait... no it's not. There's some drama and a standoff between the maintainers of flake8 and importlib_metadata. Sigh.

[dirk-thomas]

Thanks for the patch.

I just rebased it after addressing the unrelated CI failures in #53.