CompatHelper: bump compat for "DocStringExtensions" to "0.9"

[github-actions]

This pull request changes the compat entry for the DocStringExtensions package from 0.8 to 0.8, 0.9.

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 #404 (cb3f38a) into master (2aa0bea) will not change coverage.
The diff coverage is n/a.

❗ Current head cb3f38a differs from pull request most recent head 60437e2. Consider uploading reports for the commit 60437e2 to get more accurate results

@@           Coverage Diff           @@
##           master     #404   +/-   ##
=======================================
  Coverage   84.31%   84.31%           
=======================================
  Files          38       38           
  Lines        9546     9546           
=======================================
  Hits         8049     8049           
  Misses       1497     1497           

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