Update sbt-doctest with ScalaTest 3.1.0 and Dotty fixes

[travisbrown]

Scala Steward would do this eventually, but I was just trying it out anyway.

(I've verified that this update makes it possible to run the cats-kernel doctests on Dotty without warnings.)

[codecov-io]

Codecov Report

Merging #3268 into master will decrease coverage by 0.3%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3268      +/-   ##
==========================================
- Coverage   93.38%   93.08%   -0.31%     
==========================================
  Files         372      376       +4     
  Lines        7241     7428     +187     
  Branches      183      196      +13     
==========================================
+ Hits         6762     6914     +152     
- Misses        479      514      +35

Flag	Coverage Δ
#scala_version_212	93.41% <ø> (+0.02%)	⬆️
#scala_version_213	92.85% <ø> (?)

Impacted Files	Coverage Δ
...e/src/main/scala-2.13+/cats/data/ZipLazyList.scala	77.77% <0%> (ø)
...src/main/scala-2.13+/cats/instances/lazyList.scala	100% <0%> (ø)
....13+/cats/kernel/instances/LazyListInstances.scala	100% <0%> (ø)
.../main/scala-2.13+/cats/data/NonEmptyLazyList.scala	62.5% <0%> (ø)
...cala/cats/kernel/instances/FunctionInstances.scala	100% <0%> (+3.03%)	⬆️
...in/scala/cats/data/AbstractNonEmptyInstances.scala	100% <0%> (+3.44%)	⬆️
testkit/src/main/scala/cats/tests/Helpers.scala	100% <0%> (+4%)	⬆️

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