CompatHelper: bump compat for "Nemo" to "0.18"

[github-actions]

This pull request changes the compat entry for the Nemo package from 0.17 to 0.17, 0.18.

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 #3 (fa1c4b3) into master (878bc17) will decrease coverage by 0.76%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master       #3      +/-   ##
==========================================
- Coverage   89.99%   89.22%   -0.77%     
==========================================
  Files          12       12              
  Lines        1159     1077      -82     
==========================================
- Hits         1043      961      -82     
  Misses        116      116              

Impacted Files	Coverage Δ
src/utils.jl	50.00% <0.00%> (-7.15%)	⬇️
src/crystaloperator.jl	78.81% <0.00%> (-2.00%)	⬇️
src/multiplier.jl	91.30% <0.00%> (-1.80%)	⬇️
src/gallery.jl	83.41% <0.00%> (-0.48%)	⬇️
src/operatorcomposition.jl	97.43% <0.00%> (-0.19%)	⬇️
src/lattice.jl	98.83% <0.00%> (-0.16%)	⬇️
src/show.jl	100.00% <0.00%> (ø)
src/crystal.jl	100.00% <0.00%> (ø)
src/analysis.jl	100.00% <0.00%> (ø)
src/nemo_wrapper.jl	100.00% <0.00%> (ø)

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