DEPR: Deprecate use of strings denoting units with 'M', 'Y' or 'y' in pd.to_timedelta (36666)
- [x] closes #36666
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Do we only want to deprecate variations with a space (`pd.to_timedelta("1 Y")`) or also without a space  (`pd.to_timedelta("1Y")`) Ah I see some tests are failing. I will update the code. All tests related to `pd.to_timedelta` are passing. The failed tests originate from another issue on master. Updated my code and merged master. Everything passes now. I found out that the following was also possible: `pd.to_timedelta(["1Y"])` or `pd.to_timedelta(pd.Series(["1Y"]))`. So I updated `to_timedelta` to also give depr warnings in this situation. I'm not completely happy with this added code, so any feedback is appreciated Thanks @jreback , moved the warning to timedelta.pyx and the code is much cleaner now. @jreback implemented all suggestions, could you have a 2nd look? CI is still failing after merging master, but that is related to other issues than introduced in this PR. CI failed due to FATAL ERROR: Committing semi space failed. Allocation failed - process out of memory. Can someone please restart it? Thanks!
 why is 'm' (lowercase) m deprecated? this is minutes as in not ambiguous.  (agree on Y, y, M) My bad, I thought we wanted to deprecate all variations of m (lowercase and uppercase). Will revert it then. > My bad, I thought we wanted to deprecate all variations of m (lowercase and uppercase). Will revert it then.

kk great, otherwise lgtm. ping  on green. @jreback reverted the depr of lowercase m, merged master and CI on green. thanks @avinashpancham very nice!