Support up to pandas 3

[pmrv]

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Description of Changes

Widens the pandas requirement in setup.py from pandas<=2.0 to pandas>=2,<4, covering the whole pandas 2.x and 3.x series.

The codebase was audited for pandas 3 blockers; none were found:

No use of APIs removed in pandas 2/3 (DataFrame.append, applymap, Series.iteritems, delim_whitespace, .ix).
No chained assignment — all DataFrame writes go through df[col] = ... or df.loc[mask, col] = ..., so pandas 3's enforced Copy-on-Write does not change behavior.
All .map() calls operate on Series/Index objects, so nothing depends on DataFrame.map (introduced in 2.1), keeping the 2.0 lower bound valid.
Legacy pickled reference DataFrames (tests/*.pckl.gzip, [data/exmpl_df.pckl.gzip](https://github.com/pmrv/python-ace/blob/claude/sleepy-dijkstra-luopd2/data/exmpl_df.pckl.gzip)) load correctly under pandas 3, with object dtype preserved.
The existing numpy<=1.26.4 pin satisfies pandas 3's numpy ≥1.26 requirement.

Also installs the vendored lib/maxvolpy explicitly in the test workflow: pip install . does not run the custom setup.py install hook, so test_activelearning/test_activeexploration failed collection in CI with ModuleNotFoundError: maxvolpy.

Checklist

• Code is well-documented.
• All tests have been run and passed.
• Relevant documentation has been updated if necessary.

License Agreement

By submitting this pull request, I agree that:

• The code submitted in this pull request will be distributed under the Academic Software License (free for academic non-commercial use, not free for commercial use), see LICENSE.md for more details.
• The copyright for the code, including the submitted code, remains with Ruhr University Bochum. Ruhr University Bochum retains the right to transfer or modify the copyright.

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Thank you for your contribution!

[pmrv]

There's some drive-by fixes for the CI too. Not sure if those were only important for my repo or also here.