Build wheels for Python 3.10

[earwig]

Looks like no code changes are required, since the test suite passes with no issues.

I've updated the GH Actions to build Linux and macOS wheels for 3.10, but full integration is blocked on:

• appveyor/ci#3741 for Windows
• https://travis-ci.community/t/add-python-3-10/12220 for Travis CI

This might be a good opportunity to consider consolidating these services. I think we can use Actions for Windows and running tests? Maybe cibuildwheel is the way to go? I'm not sure.

I also dropped support for EOL 3.5.

[legoktm]

I forgot we were still using Travis - let's get rid of it: #279.

[hugovk]

I've updated the GH Actions to build Linux and macOS wheels for 3.10, but full integration is blocked on:

• Python 3.10 support appveyor/ci#3741 for Windows
• travis-ci.community/t/add-python-3-10/12220 for Travis CI

Now available on AppVeyor: https://github.com/appveyor/ci/issues/3741#issuecomment-958837889

No response at all from Travis CI.