Change deprecated Set operations on pd.Index

[milodubois]

Future versions of Pandas will not support pd.Index operations like this:

first | second

but must be used like this

first.union(second)

This is similar for & and ^.

See merged PR #37374 in pandas-dev/pandas.

Thus, all references, including the introductory reference in chapter 03.01 should be updated in PDSH or perhaps users should be made aware of this change when going through PDSH.

Thank you.