Update scala to 3.0.0-RC1 (#357)

Author: Philippus

.travis.yml

@@ -5,7 +5,7 @@ import: scala/scala-dev:travis/default.yml
 language: scala
 
 scala:
-  - 3.0.0-M3
+  - 3.0.0-RC1
   - 2.11.12
   - 2.12.13
   - 2.13.4

build.sbt

@@ -15,7 +15,7 @@ lazy val parserCombinators = crossProject(JVMPlatform, JSPlatform, NativePlatfor
     libraryDependencies += "com.novocode" % "junit-interface" % "0.11" % Test,
     // so we can `@nowarn` in test code, but only in test code, so the dependency
     // doesn't leak downstream
-    libraryDependencies += "org.scala-lang.modules" %% "scala-collection-compat" % "2.4.1" % Test,
+    libraryDependencies += "org.scala-lang.modules" %% "scala-collection-compat" % "2.4.2" % Test,
 
     apiMappings ++= scalaInstance.value.libraryJars.collect {
       case file if file.getName.startsWith("scala-library") && file.getName.endsWith(".jar") =>
@@ -43,7 +43,7 @@ lazy val parserCombinators = crossProject(JVMPlatform, JSPlatform, NativePlatfor
     }),
     Compile / doc / scalacOptions ++= {
       if (isDotty.value)
-        Seq()  // TODO see what flags might be desirable to pass to Scala3doc
+        Seq()  // TODO see what flags might be desirable to pass to Scala 3's Scaladoc
       else
         Seq(
           "-diagrams",