refactor(dependencies): support numpy 2

[wpbonelli]

Remove numpy upper bound to close #2153. Also fix @requires_pkg usage in test_export.py, replace deprecated itemset in mfdatastorage.py, and bump some CI jobs to Python 3.9.

This PR required updates on the MF6 side:

• https://github.com/MODFLOW-USGS/modflow6/pull/1886
• https://github.com/MODFLOW-USGS/modflow6/pull/1887

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 74.0%. Comparing base (18dfcb0) to head (ee99533). Report is 1 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #2241 +/- ## ========================================= + Coverage 73.5% 74.0% +0.4% ========================================= Files 294 294 Lines 58888 58886 -2 ========================================= + Hits 43329 43595 +266 + Misses 15559 15291 -268 ``` | [Files](https://app.codecov.io/gh/modflowpy/flopy/pull/2241?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modflowpy) | Coverage Δ | | |---|---|---| | [flopy/mf6/data/mfdatastorage.py](https://app.codecov.io/gh/modflowpy/flopy/pull/2241?src=pr&el=tree&filepath=flopy%2Fmf6%2Fdata%2Fmfdatastorage.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modflowpy#diff-ZmxvcHkvbWY2L2RhdGEvbWZkYXRhc3RvcmFnZS5weQ==) | `71.6% <100.0%> (ø)` | | ... and [11 files with indirect coverage changes](https://app.codecov.io/gh/modflowpy/flopy/pull/2241/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modflowpy)