support pytest 5.0.0+ exit codes

[mikaelarguedas]

Pytest moved their exit code to an Enum class named ExitCode in 5.0.0 and up: pytest-dev/pytest@2b92fee#diff-7191acb1a6aaab2f77b6e53ce837e348

Without this change, colcon fails to run pytests with the following error:

[4.875s] ERROR:colcon.colcon_core.task.python.test:Exception in Python testing step extension 'pytest': cannot import name 'EXIT_TESTSFAILED'
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/dist-packages/colcon_core/task/python/test/__init__.py", line 75, in test
    return await extension.step(self.context, env, setup_py_data)
  File "/usr/local/lib/python3.6/dist-packages/colcon_core/task/python/test/pytest.py", line 157, in step
    from _pytest.main import EXIT_TESTSFAILED
ImportError: cannot import name 'EXIT_TESTSFAILED'

Edit: AFAICT test failures appear unrelated to this change and are present on master cron jobs

Fixes #196.

[dirk-thomas]

Thank you for the patch.

Can you please invert the try / except logic so that the try block contains the code used moving forward and the except block contains the fallback code for earlier pytest versions.

I edited the original description to reference the issue this PR closes.

[dirk-thomas]

AFAICT test failures appear unrelated to this change and are present on master cron jobs

Actually the failing cron jobs for master are all due to the pytest 5.0 release. Since this needs additional fixes I have created #200 which includes the changes from this PR. Therefore I will close this as a duplicate.

Anyway thank you for creating the patch.

[mikaelarguedas]

Sounds good, glad it helped getting the compatibility fix in