CPython 3.7 unaccessible from `x86_64` linux

[trappitsch]

The available python versions provide 3.7 without a PYAPP_DISTRIBUTION_VARIANT. However, if the OS is linux and arch is x86_64, this variant cannot be accessed at all. In this case, if the PYAPP_DISTRIBUTION_VARIABLE is empty, it defaults to v3 -> which is not available. However, there seems to be no setting that the variable can be set to in order to access the variant "" that is the list.

This seems to be only an issue for 3.7.

Some possible fixes:

• drop 3.7 support
• ignore the PYAPP_DISTRIBUTION_VARIANT variable when 3.7 is selected, e.g., by changing the else if in the if statement in line 311 in build.rs to:

  if variant.is_empty() {
    if selected_platform == "windows" {
        variant = "shared".to_string();
    } else if selected_platform == "linux" && selected_arch == "x86_64" && selected_python_version != "3.7" {
        variant = "v3".to_string();
    }
  };

In addition, I think an overview page in the docs on what pythons are available in what configuration would be helpful. This could be auto-generated from the update_distributions.py script for example.

Again, happy to contribute these changes :)