REF: make pytables.IndexCol.itemsize a property
Along with #30074 this is a bite-sized-pieces part of a series of PRs that ends with IndexCol attributes being set in the constructor and not mutated elsewhere
Workflow related question: in your PRs that involve just pytables.py code, can you indicate this in the title? (eg to know this is not about eg Series.itemsize) will do Thanks! @jbrockmendel ideally if you can lable with HDF5 and clean/ref as appropriate