CompatHelper: bump compat for IntervalArithmetic to 0.20, (keep existing compat)

[github-actions]

This pull request changes the compat entry for the IntervalArithmetic package from 0.16, 0.17, 0.18 to 0.16, 0.17, 0.18, 0.20.
This keeps the compat entries for earlier versions.

Note: I have not tested your package with this new compat entry.
It is your responsibility to make sure that your package tests pass before you merge this pull request.

[codecov-commenter]

Codecov Report

Merging #202 (7bc2582) into master (1a208ef) will increase coverage by 0.24%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #202      +/-   ##
==========================================
+ Coverage   74.62%   74.86%   +0.24%     
==========================================
  Files           9        9              
  Lines         528      541      +13     
==========================================
+ Hits          394      405      +11     
- Misses        134      136       +2     

Impacted Files	Coverage Δ
src/paving.jl	4.00% <0.00%> (-0.35%)	⬇️
src/code_generation.jl	97.87% <0.00%> (+0.04%)	⬆️
src/setinversion.jl	95.45% <0.00%> (+0.21%)	⬆️
src/contractor.jl	78.78% <0.00%> (+0.32%)	⬆️
src/separator.jl	66.92% <0.00%> (+0.52%)	⬆️
src/ast.jl	78.94% <0.00%> (+0.63%)	⬆️

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