build/pkgs/gmpy2: Fix install-requires after #36351

[mkoeppe]

Fixes failure seen in the CI
https://github.com/sagemath/sage/actions/runs/7284422604/job/19849779382?pr=36181#step:9:936

  [sagemath_objects-10.3.beta3] ERROR: Could not find a version that satisfies the requirement gmpy2>2.1.99 (from versions: 2.2.0a1)
  [sagemath_objects-10.3.beta3] ERROR: No matching distribution found for gmpy2>2.1.99

Tests (with other merged branches) at https://github.com/mkoeppe/sage/actions/runs/7295997767/job/19883171237

📝 Checklist

• The title is concise, informative, and self-explanatory.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation accordingly.

⌛ Dependencies

• Depends on gmp 6.3.0, mpc 1.3.1, mpfr 4.2.1, mpfrcx 0.6.3, gmpy2 2.2.0a1 #36351

[github-actions]

Documentation preview for this PR (built with commit ea6fee0; changes) is ready! 🎉

[mkoeppe]

Closing - I've absorbed it into #36351, as it came back with another work item.