Bumping Python version in Travis

[XVilka]

Python 3.5 left only in a few non-mainstream and slowly paced distributions: https://repology.org/project/python3/versions

[XVilka]

And instantly caught an error:

E   pytest.PytestDeprecationWarning: the `pytest.config` global is deprecated.  Please use `request.config` or `pytest_configure` (if you're a pytest plugin) instead.

See https://github.com/pytest-dev/pytest/blob/master/doc/en/deprecations.rst#pytestconfig-global

The pytest.config global object is deprecated. Instead use request.config (via the request fixture) or if you are a plugin author use the pytest_configure(config) hook. Note that many hooks can also access the config object indirectly, through session.config or item.config for example.

[XVilka]

@williballenthin please take a look - I added GitHub Actions too, since they are faster and allow to test also MacOS and Windows easily. I cleaned up the installation process because capstone installation process was fixed as well. The pytest.config deprecation is addressed as well. The failures remain the same as on the master: https://travis-ci.org/github/williballenthin/python-idb/jobs/548889665#L1174 You can merge as is, and if you want - remove the .travis.yml if you are satisfied with GitHub Actions.

You can see the GitHub Actions results here: https://github.com/XVilka/python-idb/runs/780001388

[williballenthin]

thank you!