Release v0.5.1

v0.5.1

WARNING: etui is still a work in progress — roughly halfway to 1.0.0.
The code is alpha quality and not ready for production use. Expect
breaking changes, rough edges, and incomplete features.

Maintenance release.

• Fix the Serial tab: connecting without a port selected raised
  TypeError: App.notify() got an unexpected keyword argument 'variant'.
  The notifications now use the correct severity keyword.

Full Changelog: v0.5.0...v0.5.1