CLN: Change isocalendar to be a method
For consistency with `Timestamp.isocalendar`, `Series.dt.isocalendar` and
`DatetimeIndex.isocalendar` should rather be methods and not attributes.

Followup of #33220, see the discussions following the merge of that PR
@mroeschke dev build was fixed in #33534 by pinning CPython.  PR rebased, all green. Thanks @mgmarino for the timely and clean PRs!