build: remove gsl as the dependency in the build

[stevenhua0320]

@sbillinge ready to review. This is specific for the build on conda-forge as gsl is not needed directly by this package. When we list this as a requirement in conda, it could not understand why we need this and the pip check would fail.
In addition, we need the namespace extension to avoid the conflict as seen in the conda-forge build failure.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.63%. Comparing base (b5a3371) to head (4c0ae3a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #27   +/-   ##
=======================================
  Coverage   81.63%   81.63%           
=======================================
  Files           3        3           
  Lines          49       49           
=======================================
  Hits           40       40           
  Misses          9        9           

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[sbillinge]

please see comment

[sbillinge]

I think we want to avoid this. We don't do it for any of our other packages that all build correctly, so I think this is not the right fix. Let's at least revert this and merge the gsl fix.