Publish for Scala 3.0.0-RC3

[SethTisue]

• merge PRs
  • add Scala 3.0.0-RC3 (and drop RC1) #220
  • fix backpublishing (copy changes from scala-xml repo) #222
• choose version number
  • let's backpublish 1.0.0-RC1, there are no real changes
• tag the release
  • https://github.com/scala/scala-java8-compat/releases/tag/v1.0.0-RC1%233.0.0-RC3
• wait for release build
  • https://travis-ci.com/github/scala/scala-java8-compat/builds/223524056
• release the staging repos
  • https://oss.sonatype.org/#stagingRepositories
• artifacts on Maven Central
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-java8-compat_3.0.0-RC3/1.0.0-RC1/
  • (not a complete list)
• post-release, bump version in README? build.sbt?
  • not needed