Publish for Scala 3.0.0-RC3

[SethTisue]

• wait for scala-collection-compat to publish
• merge PRs
  • add Scala 3.0.0-RC3 (and drop RC1) #372
• choose version number
  • 1.2.0-RC2, because we have one bugfix
• tag the release and write release notes
  • https://github.com/scala/scala-parser-combinators/releases/tag/v1.2.0-RC2
• wait for release build
  • https://travis-ci.com/github/scala/scala-parser-combinators/builds/223516232
• release the staging repos
  • https://oss.sonatype.org/#stagingRepositories
• artifacts on Maven Central
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-parser-combinators_3.0.0-RC3/1.2.0-RC2/
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-parser-combinators_3.0.0-RC2/1.2.0-RC2/
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-parser-combinators_2.13/1.2.0-RC2/
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-parser-combinators_2.12/1.2.0-RC2/
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-parser-combinators_2.11/1.2.0-RC2/
  • (not a complete list)
• any README or build.sbt edits needed?
  • none needed