Out of date pandas function calls (iteritems vs items).

[iraxter]

Describe the bug

A number of the internal sksurv functions use iteritems() on dataframes which is no longer supported by pandas 2+. Looks like pyproject.toml notes this and attempts to suppress warnings but I was still receiving warnings when using pandas 1.5.3 (code would not run on pandas 2+ versions)

Grepping the codebase looks like column.py, arffwrite.py, util.py, conftest.py, test_io.py, and test_preprocessing.py all use iteritems(), could those be changed to items() instead?

[sebp]

Currently, pandas 2 is not supported. Changing iteritems() to items() and requiring pandas >= 1.0 is certainly possible.