CompatHelper: bump compat for "FixedPointNumbers" to "0.8"

[github-actions]

This pull request changes the compat entry for the FixedPointNumbers package from 0.6, 0.7 to 0.6, 0.7, 0.8.

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 #399 into master will decrease coverage by 0.24%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #399      +/-   ##
==========================================
- Coverage   78.04%   77.80%   -0.25%     
==========================================
  Files          11       11              
  Lines         870      892      +22     
==========================================
+ Hits          679      694      +15     
- Misses        191      198       +7     

Impacted Files	Coverage Δ
src/utilities.jl	86.04% <0.00%> (-9.08%)	⬇️
src/parse.jl	94.20% <0.00%> (-2.82%)	⬇️
src/colormaps.jl	95.74% <0.00%> (-0.89%)	⬇️
src/display.jl	98.80% <0.00%> (ø)	⬆️
src/conversions.jl	96.68% <0.00%> (+0.06%)	⬆️
src/differences.jl	29.93% <0.00%> (+0.90%)	⬆️
src/algorithms.jl	61.76% <0.00%> (+1.97%)	⬆️

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[kimikage]

Perhaps we can merge this, but it might be a good idea to update ColorTypes first. The updating still takes some time.
cf. PR #403