Switch CI tests from Travis to Github Actions.

[richardbarran]

See also #19. Travis-CI will no longer be free. This PR proposes a switch to Github Actions.

Notes:

• The Github Actions workflow seems designed to work with tox so I added tox to this project.
• One of the tests failed systematically. For now I've added a skip to this test so that we can test the CI successfully, this test would need fixing/updating in a separate ticket.
• I've reused the same (almost) test matrix as in the Travis configuration. I'd be happy to send in a separate PR that updates the text matrix to current versions of Python and Django.
• An exception to the above comment is that I had to remove Python 3.4 as it breaks when used with a recent release of pip.
• The existing Travis file tested against django master, but the required file is no longer available to download. I simply removed this option - my thinking was that for a mature project like this one there is little need to test against the bleeding edge? But happy to change it if you think otherwise.

Also: coverage reports
I've tried in the past to get Github Actions to interface with Coveralls (for coverage reports) but without success. I'm not the only person having issues - see this very long thread opened in 2019 for an example and the current status is that it's still buggy.

I've used Codecov instead. The report for this PR is at https://app.codecov.io/gh/richardbarran/django-js-error-hook.
If you're happy with the switch of providers you'll have to:

• Setup a Codecov account.
• Authorize your Gihub accounts to access Codecov - see https://docs.codecov.com/docs/github-oauth-application-authorization