Add aarch64 for linux

[keitherskine]

Update the CI pipeline to build wheels for ARM64 Linux processors. As requested in #837 .

Whilst we're here, remove the push/pull_request triggers for this workflow. We don't need to build wheels every time a commit is pushed. Keep workflow_dispatch so that this workflow can be manually triggered at release time.

[keitherskine]

Had to make a couple of changes to get the tests to pass.

• In Github Actions, ubuntu-latest is now 22.04 which isn't available for Python 3.6, so pinned the runner to 20.04 for the unit tests.
• In AppVeyor, for some reason I still don't understand, downloading MySQL drivers no longer works with the standard PowerShell command Invoke-WebRequest (getting 403's). Downloading drivers for SQL Server and Postgres drivers is still OK, just not MySQL. Weird. Using curl seems to work so implemented that instead. curl also has the advantage of having the --verbose option which Invoke-WebRequest doesn't have.

As well as these two fixes, I've added various other tweaks to make the AppVeyor log output better, and to make sure the AppVeyor cache of driver msi files for the jobs is kept up-to-date and correct.

This PR should be good to review now.