DEPR: Timedelta with passed unit M, Y

[jorisvandenbossche]

In [7]: pd.Timedelta(1, unit='Y')
Out[7]: Timedelta('365 days 05:49:12')

In [8]: pd.Timedelta(1, unit='M')
Out[8]: Timedelta('30 days 10:29:06')

The above does not make much sense, so lets deprecate this (similar for to_timedelta).

For the case where those unit abbreviations are used in a string, Y already raises, while M is actually interpreted as minutes:

ValueError: invalid abbreviation: Y

In [13]: pd.to_timedelta('1M')
Out[13]: Timedelta('0 days 00:01:00')

xref #16335, #12691 and #14024

[jorisvandenbossche]

One question is whether we should also deprecate 'W' for week (see #14024 (comment)).

Given that this can be easily represented as 7 days, it would be easier/clearer to just have 'D' as the highest allowed unit.
On the other hand, 'W' currently gives a sensible output (as opposed to M/Y), so less reason to remove this behaviour:

In [15]: pd.to_timedelta(1, unit='W')
Out[15]: Timedelta('7 days 00:00:00')