fix: also require kerchunk for test_read_netcdf3

[TomNicholas]

Summary

• test_read_netcdf3 only had @requires_scipy, but NetCDF3Parser.__call__ lazily imports kerchunk.netCDF3. In any environment that has scipy without kerchunk, the test raised ModuleNotFoundError instead of being skipped. Surfaced by the min-deps-build job on Dmrpp migration #902 once a transitive dep started pulling in scipy there. Adds @requires_kerchunk.

Test plan

• CI min-deps-build passes (scipy installed, kerchunk not — test should now skip).
• CI full test envs still run the test (both scipy and kerchunk present).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.04%. Comparing base (abd7d0f) to head (35d4c38).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #998      +/-   ##
==========================================
+ Coverage   89.98%   90.04%   +0.06%     
==========================================
  Files          33       33              
  Lines        2066     2089      +23     
==========================================
+ Hits         1859     1881      +22     
- Misses        207      208       +1     

see 2 files with indirect coverage changes

🚀 New features to boost your workflow:

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