Back-publish 2.0.0-M3 for Scala 3.0.0-M3

[SethTisue]

• merge a 3.0.0-M3 PR
  • add Scala 3.0.0-M3 to crossbuild #478
• choose version number
  • I'm assuming trivial-or-none code changes will be required, so that back-publishing would make sense this time. (discussion on Release 2.0.0-M3 (motive: to add Scala 3.0.0-M2 support) #471)
• tag the release
  • https://github.com/scala/scala-xml/releases/tag/v2.0.0-M3%233.0.0-M3
• wait for release build
  • https://travis-ci.com/github/scala/scala-xml/builds/209889184
• release the staging repos
• artifacts on Maven Central
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-xml_3.0.0-M3/2.0.0-M3/
• any README edits needed?
  • no