DEPR: lowercase freqs 'ye', 'qe', etc. raise a ValueError

[natmokval]

xref #56346, #56847, #9586

Using lowercase frequencies such as ‘y’, ‘ye’, ‘q’, ‘qe’, etc. raise aValueError on main, though it worked before. Using those was never documented, but if people are use these lowercase frequencies, we should keep them work.

For example:

>>> pd.period_range("2020-01-01", "2020-08-01", freq="y")
ValueError: Invalid frequency: y, failed to parse with error message: ValueError("Invalid frequency: Y, failed to parse with error message: KeyError('Y')")

>>> pd.date_range("1/1/2000", periods=4, freq="qe")
ValueError: Invalid frequency: qe, failed to parse with error message: ValueError("Invalid frequency: qe, failed to parse with error message: KeyError('qe')")

The definition of to_offset is corrected, so now we can see the FutureWarning.

>>> pd.period_range("2020-01-01", "2020-08-01", freq="y")
<stdin>:1: FutureWarning: 'y' is deprecated and will be removed in a future version, please use 'Y' instead.
PeriodIndex(['2020'], dtype='period[Y-DEC]')

>>> pd.date_range("1/1/2000", periods=4, freq="qe")
<stdin>:1: FutureWarning: 'qe' is deprecated and will be removed in a future version, please use 'QE' instead.
DatetimeIndex(['2000-03-31', '2000-06-30', '2000-09-30', '2000-12-31'], dtype='datetime64[ns]', freq='QE-DEC')

• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.

[natmokval]

@MarcoGorelli, could you please take a look at this PR?

[MarcoGorelli]

Just in time - well done for spotting this and fixing it so close to the release!

Looking forwards to enforcing these deprecations so that all this complexity can be reduced

Nice one @natmokval !

[natmokval]

Thank you for reviewing this PR!

[lithomas1]

@MarcoGorelli is this scheduled for removal in 4.0?

you might need to manually move the deprecation bullet from the 4.0 tracker issue to the 3.0 tracker issue if it isn't.

[MarcoGorelli]

thanks, will do