backport py3 testing enhancements

[williballenthin]

backport py3 testing enhancements from #234. basically: wrap py2-only things in xfail annotations.

when we add a py3 backend, then all we need to do is update fixtures::get_extractor() to return the py3 backend on py3 and then re-run the tests. pytest will complain that things that we used to fail no longer fail, so go through and remove those xfail marks.

[williballenthin]

🚀

[williballenthin]

we might want to merge this routine with main.get_extractor() at some point.

[mr-tz]

very good!

[Ana06]

LGTM 👍 👍

[Ana06]

This could be done with a matrix for the python version instead of duplicating the code. But as we want to get rid of Python 2 in the near future, I guess this won't have a long life anyway... 😄

[williballenthin]

oh, good point. though, then the linting would happen twice?

[Ana06]

@williballenthin

though, then the linting would happen twice?

no, I meant changing tests to the following (instead of duplicating tests into tests27 and test38:

  tests:
    name: Test in ${{ matrix.python }}
    runs-on: ubuntu-latest
    needs: [code_style, rule_linter]
    strategy:
      matrix:
        include:
          - python: 2.7
          - python: 3.8
    steps:
    - name: Checkout capa with submodules
      uses: actions/checkout@v2
      with:
        submodules: true
    - name: Set up Python ${{ matrix.python }}
      uses: actions/setup-python@v2
      with:
        python-version: ${{ matrix.python }}
    - name: Install capa
      run: pip install -e .[dev]
    - name: Run tests
      run: pytest tests/

I can send a PR as this can be useful if we want to test several python versions.