Scala 2.13.10 (was 2.13.8)

includes a small tweak necessary in order to adapt to
scala/scala#10047

this originally came up at
scala/community-build#1579

Author: SethTisue

.github/workflows/ci.yml

@@ -10,7 +10,7 @@ jobs:
       fail-fast: false
       matrix:
         java: [8, 11, 17]
-        scala: [2.13.8, 3.1.3]
+        scala: [2.13.x, 3.x]
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2

build.sbt

@@ -1,5 +1,5 @@
 ThisBuild / scalaVersion := "3.1.3"
-ThisBuild / crossScalaVersions := Seq((ThisBuild / scalaVersion).value, "2.13.8")
+ThisBuild / crossScalaVersions := Seq((ThisBuild / scalaVersion).value, "2.13.10")
 
 lazy val root = project.in(file("."))
   .aggregate(collectionContrib.jvm, collectionContrib.js)

src/main/scala/scala/collection/MultiDict.scala

@@ -198,7 +198,7 @@ trait MultiDictOps[K, V, +CC[X, Y] <: MultiDict[X, Y], +C <: MultiDict[K, V]]
   def filterSets(p: ((K, Set[V])) => Boolean): C =
     fromSpecificSets(new View.Filter(sets, p, isFlipped = false))
 
-  override def addString(sb: StringBuilder, start: String, sep: String, end: String): StringBuilder =
+  override def addString(sb: StringBuilder, start: String, sep: String, end: String): sb.type =
     iterator.map { case (k, v) => s"$k -> $v" }.addString(sb, start, sep, end)
 }