chore(dependencies): support pandas 3

[wpbonelli]

Recently pandas 3 was released.

Only minimal changes needed from us, namely

• switching timedelta units from deprecated "d" to "D"
• a fix in the model splitter where columns that were previously renamed automatically on dataframe construction must now be renamed manually due to stricter dataframe init method requirements

There are two other potentially relevant changes

1. String dtype

Pandas 3 will infer string columns as str dtype instead of object. Code checking dtype == 'object' for strings will break. I think we are safe as internal dtype checks are generally on numpy arrays, not pandas DataFrames. We kind of luck out for having not migrated everything over to pandas.

When we do pd.DataFrame.from_records(recarray), numpy string fields will get converted to str dtype instead of object. But I don't think we need to care, indexing, selection, etc should all work as before.

2. Copy-on-Write (CoW)

As far as I can tell we use .loc internally, no chained assignments, so we should be good.

[codecov]

Codecov Report

❌ Patch coverage is 58.33333% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.4%. Comparing base (556c088) to head (33f590e).
⚠️ Report is 136 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/mf6/data/mfdataplist.py	60.0%	4 Missing ⚠️
flopy/export/metadata.py	0.0%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2723      +/-   ##
===========================================
+ Coverage     55.5%    72.4%   +16.9%     
===========================================
  Files          644      674      +30     
  Lines       124135   130560    +6425     
===========================================
+ Hits         68947    94586   +25639     
+ Misses       55188    35974   -19214     

Files with missing lines	Coverage Δ
flopy/utils/mtlistfile.py	88.9% <100.0%> (ø)
flopy/export/metadata.py	23.2% <0.0%> (ø)
flopy/mf6/data/mfdataplist.py	71.6% <60.0%> (-7.2%)	⬇️

... and 569 files with indirect coverage changes

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