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

[github-actions]

This pull request changes the compat entry for the Polynomials 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 #158 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #158   +/-   ##
=======================================
  Coverage   93.45%   93.45%           
=======================================
  Files           4        4           
  Lines         336      336           
=======================================
  Hits          314      314           
  Misses         22       22           

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