Unable to depend on v3.1.0

[Matt Palmer]

Following the release of v3.1.0 you are unable to import the project when using Go modules.

Previously in the v3.0.0 release, the addition of +incompatible within the go.mod would allow importing as this project was not a go module.

However, now that the project contains a go.mod you are unable to use the +incompatible functionality to import the project.

See releasing-modules-v2-or-higher for the options on how to handle this.