Fix usage of deprecated startdir parameter

[suutari-ai]

Use the start_path parameter instead of startdir, since Pytest 7 yield the following deprecation warning if startdir is being used:

site-packages/pytest_sugar.py:279: PytestRemovedIn8Warning:
The (startdir: py.path.local) argument is deprecated, please
use (start_path: pathlib.Path)

Details of the Pytest deprecation can be found from: https://docs.pytest.org/en/latest/deprecations.html#py-path-local-arguments-for-hooks-replaced-with-pathlib-path

Fixes #232

[justinmayer]

This change causes a test failure with one of the existing Pytest 6.2.x-based workflows, so we would either need to change the code to support both 6.x and 7.x, or else drop support for Pytest <7.0. My preference would be to support backwards compatibility in this case.

[suutari-ai]

Thanks for the comments!

I updated the PR to use startdir for pytest 6 or lower and start_path otherwise. Tests should now pass with both pytest 6 and 7. Also tested with pytest 8.0.0rc2 and it seems to work too which is an improvement to the previous code, which failed with pytest 8.

[justinmayer]

@suutari-ai: Many thanks for the follow-up improvements to your pull request. Much appreciated! I'll merge this now with the intention of shipping this fix at a later date as pytest-sugar 1.0.0 once Pytest 8.0 has been released. That imminent pytest-sugar 1.0.0 release will also serve as a good opportunity to officially drop Python 3.7 support.