[FEA] Series datetime isocalendar

[beckernick]

As noted in #4908 , pandas plans to eventually deprecate series.dt.week and series.dt.weekofyear in favor of series.dt.isocalendar().week. The isocalendar() method returns a DataFrame containing the ISO 8601 year, week of year (number), and day of week (1-7, rather than 0-6 like series.dt.dayofweek).

It also exists on the Timestamp class, on which it returns a tuple of the three values. This use case is generally less common.

This feature should probably come after #4908 , as weekofyear | week were marked for deprecation in pandas 1.1 but are still alive and kicking in 1.3.

import pandas as pd
​
s = pd.Series(pd.date_range(start="2021-07-25", end="2021-07-30"))
print(s.dt.isocalendar())
   year  week  day
0  2021    29    7
1  2021    30    1
2  2021    30    2
3  2021    30    3
4  2021    30    4
5  2021    30    5

import pandas as pd
pd.Timestamp("2021-07-01").isocalendar()
(2021, 26, 4)

[marlenezw]

This issue should be pretty easily solved once #9047 is merged! Looks like it's still waiting on reviews.