Switch CI to Github Actions

[richardbarran]

See also #64. Travis-CI will no longer be free. This PR proposes a switch to Github Actions.

Notes:

• I removed testing against Python 2.7 as it required special cases in the tests (e.g. installing a specific version of django-honeypot) and 2.7 has been deprecated for a long time, so we shouldn't be supporting it anyway, I think.
• The existing tox 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.
• I've set badges in the README to point to my own repo (so that you can see that they're working). They'll need updating after a merge to point back to the master branch.

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-envelope.
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
• The Codecov badge sourcecode can be found in settings/badge.

The test matrix is outdated, but I've left it 'as is' as much as possible - with the exception of removing Python 2.7. I'd be happy to submit a second PR with updates to test only supported versions of Django e.g. 3.2 and of Python.

[richardbarran]

Addition to above: I notice that the docs has a development page that would need updating with correct links to the new CI pages.