tethys v0.26.0

Features

• Add ability to add discriminator to JsonObjectWriter @ulanzetz (#142)

Dependencies

• Bump actions/cache from 2.1.6 to 2.1.7 @dependabot (#136)
• Update Scala, sbt and dependencies @nikolaev-nikita, @scala-steward (#143, #145 )
• Update scalatest-flatspec, ... to 3.2.10 @scala-steward (#133)