Publish for Scala 3.0.0-RC3

[SethTisue]

• merge PRs
  • add Scala 3.0.0-RC3 (and drop RC1) #509
• choose version number
  • there are no real changes, so let's backpublish 2.0.0-RC1
• tag the release
  • https://github.com/scala/scala-xml/releases/tag/v2.0.0-RC1%233.0.0-RC3
• wait for release build
  • https://travis-ci.com/github/scala/scala-xml/builds/223518914
• release the staging repos
  • https://oss.sonatype.org/#stagingRepositories
• artifacts on Maven Central
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-xml_3.0.0-RC3/2.0.0-RC1/
  • (not a complete list)
• post-release, bump version in README and build.sbt
  • not needed