Update to scala 3 M3 (#314)

Author: johnynek

build.sbt

@@ -2,7 +2,7 @@ import sbtcrossproject.{crossProject, CrossType}
 
 val Scala212 = "2.12.12"
 val Scala213 = "2.13.4"
-val Scala3 = "3.0.0-M1"
+val Scala3 = "3.0.0-M3"
 
 ThisBuild / crossScalaVersions := Seq(Scala213, Scala212)
 ThisBuild / scalaVersion := Scala213