CLN: enforce deprecation of frequencies deprecated for offsets
xref #52064, #55792, #55553, #55496
Enforced deprecation of aliases ``M``, ``Q``, ``Y``, etc. in favour of ``ME``, ``QE``, ``YE``, etc. for offsets. Now the aliases are case-sensitive.

P.S. Corrected a note in v3.0.0 related to PR #57627
I enforced deprecation of aliases `M, Q, Y, etc.` in favour of `ME, QE, YE, etc.` for offsets, ci - green. @MarcoGorelli could you please take a look at this PR? /preview Website preview of this PR available at: https://pandas.pydata.org/preview/pandas-dev/pandas/57986/ > Thanks for updating! A few things
> 
>     * good to have the `c_PERIOD_TO_OFFSET_FREQSTR` dict, but I don't see why it's being used here:
>       ```python
>       if not is_period:
>           if name.upper() in c_OFFSET_REMOVED_FREQSTR:
>               raise ValueError(
>                   f"\'{name}\' is no longer supported for offsets. Please "
>                   f"use \'{c_OFFSET_REMOVED_FREQSTR.get(name.upper())}\' "
>                   f"instead."
>               )
>           # below we raise for lowrecase monthly and bigger frequencies
>           if (name.upper() != name and
>                   name.lower() not in {"h", "min", "s", "ms", "us", "ns"} and
>                   name.upper() not in c_PERIOD_TO_OFFSET_FREQSTR and
>                   name.upper() in c_OFFSET_TO_PERIOD_FREQSTR):
>               raise ValueError(INVALID_FREQ_ERR_MSG.format(name))
>       ```
>     
>       If you've already checked `if not is_period`, when why do you need to check if it's in `c_PERIOD_TO_OFFSET_FREQSTR `?
> 
we need the check if it isn't in `c_PERIOD_TO_OFFSET_FREQSTR`, because we did not deprecate lowercase frequencies `"d", "b", "w", "weekday", "w-sun”,` and so on. We don't want to raise a ValueError for these frequencies for both offsets and period. After deprecating these frequencies we can remove the check (only uppercase will be correct).

for example without this check `test_reindex_axes` in `pandas/tests/frame/methods/test_reindex.py` raises a ValueError for `freq = "d"` >     * `lowrecase` typo 
thanks, I corrected the typo >     * is this part temporary
>       ```python
>         elif name in {"d", "b"}:
>           name = name.upper()
>       elif (name.upper() not in {"B", "D"} and
>               not name.upper().startswith("W")):
>       ```
>       If so, could you add a comment explaining why it needs to be there, possibly linking to an open issue? Ideally we should get to the point where we can get rid of all this complexity, so let's make it clear what the road towards that endpoint is
Yes, it's the temporary part. I left the comment below.
 thanks for explaining - is there a way to do that part without using `c_PERIOD_TO_OFFSET_FREQSTR`? let's try to separate them out a bit more, if `c_PERIOD_TO_OFFSET_FREQSTR` is for mapping period aliases to offset aliases, then we probably shouldn't be using it for offset aliases > thanks for explaining - is there a way to do that part without using `c_PERIOD_TO_OFFSET_FREQSTR`? let's try to separate them out a bit more, if `c_PERIOD_TO_OFFSET_FREQSTR` is for mapping period aliases to offset aliases, then we probably shouldn't be using it for offset aliases

but what should we do with aliases which are the same for both: period and offsets, such as `"D", "B", "W", "WEEKDAY", "W-SUN”`, etc.?
I think we need to keep them in both dictionaries `c_PERIOD_TO_OFFSET_FREQSTR` and `c_OFFSET_TO_PERIOD_FREQSTR`, we use them then we check case correctness. Oops, I forgot  to add "WEEKDAY" in `c_PERIOD_TO_OFFSET_FREQSTR`, should I add it?

After enforcing the deprecation of `"d", "b", "w", "weekday", "w-sun”`, etc. we can make simplifications. > but what should we do with aliases which are the same for both: period and offsets, such as "D", "B", "W", "WEEKDAY", "W-SUN”, etc.?
I think we need to keep them in both dictionaries c_PERIOD_TO_OFFSET_FREQSTR and c_OFFSET_TO_PERIOD_FREQSTR, we use them then we check case correctness.

I'd suggest either that, or to add a set which contains aliases which are valid for both > > but what should we do with aliases which are the same for both: period and offsets, such as "D", "B", "W", "WEEKDAY", "W-SUN”, etc.?
> > I think we need to keep them in both dictionaries c_PERIOD_TO_OFFSET_FREQSTR and c_OFFSET_TO_PERIOD_FREQSTR, we use them then we check case correctness.
> 
> I'd suggest either that, or to add a set which contains aliases which are valid for both

thanks, then maybe we can leave it as it is?  cool, I think this is on the right track

I think there's a logic error somewhere, as it currently gives
```
In [7]: pd.period_range('2000', periods=3, freq='s')
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
File offsets.pyx:4853, in pandas._libs.tslibs.offsets.to_offset()

ValueError: 's' is not supported as period frequency.

During handling of the above exception, another exception occurred:

ValueError                                Traceback (most recent call last)
Cell In[7], line 1
----> 1 pd.period_range('2000', periods=3, freq='s')

File ~/pandas-dev/pandas/core/indexes/period.py:585, in period_range(start, end, periods, freq, name)
    582 if freq is None and (not isinstance(start, Period) and not isinstance(end, Period)):
    583     freq = "D"
--> 585 data, freq = PeriodArray._generate_range(start, end, periods, freq)
    586 dtype = PeriodDtype(freq)
    587 data = PeriodArray(data, dtype=dtype)

File ~/pandas-dev/pandas/core/arrays/period.py:321, in PeriodArray._generate_range(cls, start, end, periods, freq)
    318 periods = dtl.validate_periods(periods)
    320 if freq is not None:
--> 321     freq = Period._maybe_convert_freq(freq)
    323 if start is not None or end is not None:
    324     subarr, freq = _get_ordinal_range(start, end, periods, freq)

File period.pyx:1768, in pandas._libs.tslibs.period._Period._maybe_convert_freq()

File offsets.pyx:4914, in pandas._libs.tslibs.offsets.to_offset()

ValueError: Invalid frequency: s, failed to parse with error message: ValueError("'s' is not supported as period frequency.")
```
but 's' should definitely be supported here, right? Thanks for updating, looking better - still got a comment though, else dicts' responsibilities are being mixed

I like the good error messages you're giving here. Perhaps `pd.date_range('2000', periods=2, freq='T')` should also give a good error message, advising to use `'min'`? (as a separate pr, though) thanks! this is probably good, will do another pass over tomorrow / in the week > thanks! this is probably good, will do another pass over tomorrow / in the week

thank you for helping me with this PR! Thanks @natmokval 