Publish for Scala 3.0.0-M3

[SethTisue]

• merge the 3.0.0-M3 PR (#403)
• choose version number
  • I would prefer to backpublish 2.3.1, but publishing is weird/nonstandard in this repo (#395)
  • so, sigh, but let's call it 2.3.2
• create the release and write the notes
  • https://github.com/scala/scala-collection-compat/releases/tag/v2.3.2
• wait for release build
  • https://travis-ci.com/github/scala/scala-collection-compat/builds/209887829
• release the staging repos
• artifacts on Maven Central
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-collection-compat_3.0.0-M3/2.3.2/
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-collection-compat_2.13/2.3.2/
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-collection-compat_2.12/2.3.2/
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-collection-compat_2.11/2.3.2/
• post-release, bump version in README
  • e17f7e3
  • oops a4e0dd3