Fixed and tested for 3.8

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[snoack]

This merge request seems incomplete. It doesn't add Python 3.8 to tox.iniand .travis.yml, and the changes break on other Python versions (see the CI status).

[insignification]

@snoack - did you give up on my pull requests already? As you recall, #28 provides stable py38 support, together with stabler support for other py versions (less crashes/bugs/etc, with tests).

[rh314]

EDIT: ignore the comment below, I see the code.co_kwonlyargcount fix was already introduced here: https://github.com/snoack/python-goto/blame/acbe736221d2238df3d09beab457d0bb19d05812/goto.py#L55

OUTDATED COMMENT: Ah! Was going to create a similar PR. Discovered the same fix. (the extra code.co_kwonlyargcount parameter in 3.8)
@snoack, please consider looking at this PR again 🙏