CompatHelper: bump compat for "DataFrames" to "0.21"

[github-actions]

This pull request changes the compat entry for the DataFrames package from 0.13, 0.20 to 0.13, 0.20, 0.21.

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]

Codecov Report

Merging #12 into master will decrease coverage by 4.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #12      +/-   ##
==========================================
- Coverage   38.00%   33.92%   -4.08%     
==========================================
  Files           5        5              
  Lines          50       56       +6     
==========================================
  Hits           19       19              
- Misses         31       37       +6     

Impacted Files	Coverage Δ
src/Geometry.jl	63.63% <0.00%> (-6.37%)	⬇️
src/Bind.jl	0.00% <0.00%> (ø)
src/Interpolate.jl	0.00% <0.00%> (ø)

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