Travis-CI has been slow of late, although this might improve now we have migrated to travis-ci.com

In #591 I suggested we add support for arm by using Shippable. In #705 @karlding added arm64 support using travis-ci.
In #661 @karlding proposed moving to Azure Pipelines.
In #940 @felixdivo proposed:

• retain all Github workflow tests, which test with CPython on Linux, macOS and Windows
• Move CPython 3.10-dev & nightly from Travis to Github workflow
• remove Travis CI testing except: pypy3 and socketcan tests
• retain the other Travis jobs like the linter and publishing (for now at least, we can consider moving them in another PR)
• remove AppVeyor testing as it is entirely covered by Github workflows (right?)