Fix tests, test Python 3.10, and run on tests on GitHub Actions

[adamchainz]

Several commits, happy to split if appropriate.

1. Fix warning on Pytest

Fix:

tests/nodeenv_test.py:49
  /home/runner/work/nodeenv/nodeenv/tests/nodeenv_test.py:49: PytestDeprecationWarning: @pytest.yield_fixture is deprecated.
  Use @pytest.fixture instead; they are the same.
    @pytest.yield_fixture

2. Fix Flake8 error

Fix this error:

nodeenv.py:359:80: E501 line too long (84 > 79 characters)

Added in 8e1c574

3. Update argument parser usage in tests

Missed in 8e1c574.

4. Drop Python 3.6, add Python 3.10, run tests on GitHub Actions

Travis no longer posts results over to GitHub. I think this is how the previous failures went unseen. This commit removes the Travis config and adds a GitHub Actions one based on a recipe I use in my projects. This includes a method of combining Coverage results on GitHub Actions as per this blog post, removing the need for coveralls.

I removed Python 3.6, which is EOL, and added tests for Python 3.8 through 3.10. I didn’t remove 2.7 but would be happy to do so - it’s 2 years past its EOL.

I dropped the PyPy test run because the module is pure Python. I don’t think it’s worth the effort trying to test every library in existence with PyPy.

[adamchainz]

The test run needs approving on GitHub Actions. You can see a successful run on my fork’s actions tab: https://github.com/adamchainz/nodeenv/actions/runs/1659055175

[ekalinin]

I didn’t remove 2.7 but would be happy to do so - it’s 2 years past its EOL.

Yeah, I understand. But they're a lot of projects still on py2.7. So, let's save 2.7 support for a while.

Thank you! Great patch!

[adamchainz]

Great, thank you for fast review.