Publish for Scala 3.0.0-RC2

[SethTisue]

• merge PRs
  • Scala 3.0.0-RC2 + sbt 1.5 #440
• choose version number
  • 2.4.3
• create the release and write the notes
  • https://github.com/scala/scala-collection-compat/releases/tag/v2.4.3
• wait for release build
  • https://travis-ci.com/github/scala/scala-collection-compat/builds/221561709
• release the staging repos
  • https://oss.sonatype.org/#stagingRepositories
• artifacts on Maven Central
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-collection-compat_2.11/2.4.3/
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-collection-compat_2.12/2.4.3/
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-collection-compat_2.13/2.4.3/
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-collection-compat_3.0.0-RC1/2.4.3/
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-collection-compat_3.0.0-RC2/2.4.3/
  • (not a complete list)
• post-release, bump version in README
  • 61825f7