Publish for Scala 3.0.0-RC3

[SethTisue]

• wait for ScalaCheck to publish
• update the crossbuild
  • add Scala 3.0.0-RC3 (and drop RC1) #172
• choose version number
  • let's backpublish 1.0.2, since there are no real changes
• tag the release
  • https://github.com/scala/scala-parallel-collections/releases/tag/v1.0.2%233.0.0-RC3
• wait for release build
  • https://travis-ci.com/github/scala/scala-parallel-collections/builds/223505409
• release the staging repos
  • http://oss.sonatype.org/#stagingRepositories
• artifacts on Maven Central
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-parallel-collections_3.0.0-RC3/1.0.2/
• readme updates? build.sbt updates?
  • none needed