build: no upper bound on the Cython version

[tlambert03]

Cython >= 3.0,<3.2 (since e62d6a6, without a stated reason) means that nothing that depends on witty can be installed next to Cython 3.2 or 3.3. For waterz, which is about to make witty a hard dependency, that would make pip install waterz cython fail for anyone on a current Cython.

Testing

• witty's tests with Cython 3.3.0 on Python 3.10 and 3.14: 4 passed each.
• waterz's tests (103, including its JIT path through witty) with this branch and Cython 3.3.0 on Python 3.13: all pass.
• CI resolves the newest Cython, so it runs with 3.3.0 here.

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.73%. Comparing base (d30f274) to head (0f0ad4c).
⚠️ Report is 13 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #17      +/-   ##
==========================================
- Coverage   92.94%   92.73%   -0.21%     
==========================================
  Files           2        2              
  Lines         170      179       +9     
==========================================
+ Hits          158      166       +8     
- Misses         12       13       +1     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

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