DEPR: freq ''2BQ-SEP" for to_period should raise an error

[natmokval]

on the latest main this

>>> rng = pd.date_range('01-Jan-2012', periods=8, freq='2BQ-SEP')
<stdin>:1: FutureWarning: 'BQ-SEP' is deprecated and will be removed in a future version, please use 'BQE-SEP' instead.
>>> rng.to_period()

works as

PeriodIndex(['2012Q1', '2012Q3', '2013Q1', '2013Q3', '2014Q1', '2014Q3',
             '2015Q1', '2015Q3'],
            dtype='period[2Q-DEC]')

While before it raised an error

AttributeError: 'pandas._libs.tslibs.offsets.BQuarterEnd' object has no attribute '_period_dtype_code'