Add support for pandas 2.0

[sebp]

Checklist

• closes Out of date pandas function calls (iteritems vs items). #372
• pytest passes
• tests are included
• code is well formatted
• documentation renders correctly

What does this implement/fix? Explain your changes
Removes calls of deprecated pandas API.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01 🎉

Comparison is base (e376da4) 97.92% compared to head (fcba890) 97.94%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #373      +/-   ##
==========================================
+ Coverage   97.92%   97.94%   +0.01%     
==========================================
  Files          37       37              
  Lines        3326     3354      +28     
  Branches      502      508       +6     
==========================================
+ Hits         3257     3285      +28     
  Misses         33       33              
  Partials       36       36              

Impacted Files	Coverage Δ
sksurv/column.py	100.00% <100.00%> (ø)
sksurv/datasets/base.py	90.82% <100.00%> (+0.17%)	⬆️
sksurv/io/arffwrite.py	90.90% <100.00%> (ø)
sksurv/util.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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