Linux wheels are not tested

[riddell-stan]

Linux wheels are not tested. Testing linux wheels was removed in commit 8a10103

Linux wheels are working fine on Ubuntu 20.04. They do not work on Ubuntu 18.04. There is no way to use multibuild to test the wheels on Ubuntu 20.04.

Right now we are assuming that if the wheels pass tests on macOS they will also pass tests on Ubuntu 20.04.

[riddell-stan]

It's likely not too difficult to switch to cibuildwheel. See osmcode/pyosmium-wheel-build#14

If we switched to cibuildwheel we could eliminate this repository entirely.

[riddell-stan]

Closed in #82