add compat with distributions v0.20

[tlnagy]

fixes #1287

Contributor checklist:

• I've run tests locally

[codecov-io]

Codecov Report

Merging #1288 into master will increase coverage by 11.06%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #1288       +/-   ##
===========================================
+ Coverage   74.74%   85.81%   +11.06%     
===========================================
  Files          36       36               
  Lines        4086     4158       +72     
===========================================
+ Hits         3054     3568      +514     
+ Misses       1032      590      -442

Impacted Files	Coverage Δ
src/varset.jl	97.77% <0%> (-2.23%)	⬇️
src/coord.jl	84.53% <0%> (+1.03%)	⬆️
src/aesthetics.jl	76.85% <0%> (+3.13%)	⬆️
src/misc.jl	62.42% <0%> (+3.44%)	⬆️
src/geometry.jl	75% <0%> (+3.57%)	⬆️
src/geom/beeswarm.jl	78.75% <0%> (+3.75%)	⬆️
src/geom/hvabline.jl	93.84% <0%> (+4.61%)	⬆️
src/bincount.jl	84.53% <0%> (+5.15%)	⬆️
src/geom/violin.jl	100% <0%> (+5.26%)	⬆️
src/geom/errorbar.jl	91.89% <0%> (+5.4%)	⬆️
... and 23 more

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