Reader contravariant on input type A

[kklorenzotesta]

Kleisli is contravariant on A from #2843 , so Reader and ReaderT should also be contravariant in A.

[codecov-io]

Codecov Report

Merging #2922 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #2922   +/-   ##
=======================================
  Coverage   94.24%   94.24%           
=======================================
  Files         367      367           
  Lines        7002     7002           
  Branches      195      195           
=======================================
  Hits         6599     6599           
  Misses        403      403

Impacted Files	Coverage Δ
core/src/main/scala/cats/data/package.scala	90.9% <ø> (ø)	⬆️
core/src/main/scala/cats/data/Kleisli.scala	90.99% <100%> (ø)	⬆️

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