build flatland-rl wheel on python3.7 and expose as artifact

1. Will build wheel on all branches (TODO: use the built wheel for tests as well)
2. Final artifact and it's url will be available at https://gitlab.aicrowd.com/flatland/flatland/-/jobs/ [a download button beside the build_wheel job]
3. Artifact will be available for 2 months. This can be changed from the gitlab web ui (configurable per artifact).

[Also renames the wheel file to indicate that this will not work on python2]