DEPR: deprecate strings T, S, L, U, and N in offsets frequencies,  resolution abbreviations, _attrname_to_abbrevs
xref #52536

deprecated codes `'T'` and `'L'` in `_attrname_to_abbrevs/_abbrev_to_attrnames`, added a test for FutureWarning.

EDIT: 

- Deprecated in Timedelta units `'T', 'L', 'U', 'N'` in favour of `'min', 'ms', 'us', 'ns'`.
- Deprecated aliases `'T', 'L', 'U', 'N'` for time series frequencies in favour of `'min', 'ms', 'us', 'ns'`.
- Deprecated resolutions `'T', 'L', 'U', 'N'` for Timedelta.resolution_string  in favour of `'min', 'ms', 'us', 'ns'`.
@jbrockmendel, could you please take a look at my pr? I started deprecating codes `“T”` and `“L”` in _attrname_to_abbrevs/_abbrev_to_attrnames and I got an issue. Could you please help me with this?

First, If we deprecate `“T”` and `“L”`, we should replace them with new abbreviations. I don't know that we can use instead of  `“T”` and `“L”`. Can we use one of the abbreviations, which we have for minutes and milliseconds, e.g.: min and ms?

And another question: in class Minute(Tick) and class Milli(Tick) we use `“T”` and `“L”` as a _prefix. As I understand we deprecate them as well and we replace them with the new abbreviations for minutes and milliseconds. Am I correct? > First, If we deprecate “T” and “L”, we should replace them with new abbreviations. I don't know that we can use instead of “T” and “L”. Can we use one of the abbreviations, which we have for minutes and milliseconds, e.g.: min and ms?

> And another question: in class Minute(Tick) and class Milli(Tick) we use “T” and “L” as a _prefix. As I understand we deprecate them as well and we replace them with the new abbreviations for minutes and milliseconds. Am I correct?

Yes and yes. Hi @jbrockmendel. I have a question. Should we deprecate `'A'` for `'year'` as well? We can left `'Y'` as an alias for '`year'`. 

My qustion is related to PR: DEPR offsets: rename ‘M’ to ‘ME’ #52064. I started deprecating for offsets `'A'` in favour of `'AE' `and I can do either:

- rename A to AE and Y to YE
- deprecate A/AE in favour of Y/YE > Nice!
> 
> Have left some comments (which maybe you were going to address anyway 🙈 either way, thought I'd leave them in case)

Thanks for your comments, they are always very helpful. And thank you for helping me with this PR. Is the idea still to get in all these code deprecations (including A and M) in the same release (2.2)? they wouldn't necessarily need to go in together

my general stance is:
- if we get "M -> ME" in, then we should probably get "Y -> YE" / "Q -> QE" / etc. in at the same time
- deprecating "L" / "U" / "T" (or "A") can be done independently of the above

So, if you're happy with this (if Brock's comments are addressed in time), I think it'd be OK to get this in for 2.1 and do any others for 2.2 > So, if you're happy with this (if Brock's comments are addressed in time), I think it'd be OK to get this in for 2.1 and do any others for 2.2

Cool sgtm @MarcoGorelli, @jbrockmendel, could you please take a look at my changes? I corrected the pr as you suggested. Seems like failures in ci aren't related to my changes.  > Looks really good to me, fantastic work here! Sorry it's taken a while to review. I think this should go in
> 
> Leaving open a bit in case anyone has objections, but then will merge
> 

Thank you for helping me with this PR.
Cats are amazing :) Right, let's do this, thanks @natmokval 