Require sacc 2.1 or newer

[marcpaterno]

Description

This PR moves us to a newer version of Sacc, to get a fix for a bug that prevents writing FITS-format Sacc files containing large covariance matrices (with more than 999 columns).

Type of change

Please delete the bullet items below that do not apply to this pull request.

• Bug fix (non-breaking change which fixes an issue)

Checklist:

The following checklist will make sure that you are following the code style and
guidelines of the project as described in the
contributing page.

• I have run bash pre-commit-check and fixed any issues
• I have added tests that prove my fix is effective or that my feature works
• I have made corresponding changes to the documentation
• I have 100% test coverage for my changes (please check this after the CI system has verified the coverage)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.0%. Comparing base (58ae1b8) to head (abfa58e).
⚠️ Report is 94 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #564   +/-   ##
=======================================
  Coverage   100.0%   100.0%           
=======================================
  Files          66       66           
  Lines        5984     5984           
  Branches      703      703           
=======================================
  Hits         5984     5984           

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