Update sbt to 1.3.9

[scala-steward]

Updates org.scala-sbt:sbt from 1.3.8 to 1.3.9.
GitHub Release Notes - Version Diff

I'll automatically update this PR to resolve conflicts as long as you don't change it yourself.

If you'd like to skip this version, you can just close this PR. If you have any feedback, just mention me in the comments below.

Have a fantastic day writing Scala!

Ignore future updates

Add this to your .scala-steward.conf file to ignore future updates of this dependency:

updates.ignore = [ { groupId = "org.scala-sbt", artifactId = "sbt" } ]

labels: library-update, semver-patch

[codecov-io]

Codecov Report

Merging #336 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #336   +/-   ##
=======================================
  Coverage   86.51%   86.51%           
=======================================
  Files         135      135           
  Lines        1476     1476           
  Branches       41       41           
=======================================
  Hits         1277     1277           
  Misses        199      199           

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