PDEP0004: implementation
- [x] closes #12585 (Replace xxxx with the Github issue number)
- [x] closes #46071
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [x] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [x] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Finally ready for review - I've separated the types of changes by commit to try to make the review easier. I've also split out a few changes into precursor PRs, which have now been merged.
This changes a lot of files, but I think it's hard to split it out further

PDEP for reference: https://pandas.pydata.org/pdeps/0004-consistent-to-datetime-parsing.html
Thanks for your reviews, much appreciated!

There are still some open issues related to datetime parsing, but I don't think any of them should be blockers to this:
- #50072
- #50071
- #50051
- #50025

I've addressed the previous blocker, so it'd be good if we could move this along @jbrockmendel @mroeschke  just a couple of comments Thanks all! @Dr-Irv there's still a "requested changes" from you - any further comments? > Thanks all! @Dr-Irv there's still a "requested changes" from you - any further comments?

I think there are 2 things that I'd like clarification on (listing them here so I don't have to find them again):

- https://github.com/pandas-dev/pandas/pull/49024/files#r1046027911
- https://github.com/pandas-dev/pandas/pull/49024/files#r1046036247 > I'm good now - all issues resolved.

Thanks! Really appreciate all the reviews here

That's 4 explicit approvals from core members, unless there's objections I'll merge soonish then - there's already 110+ messages here and it's getting hard to navigate

If minor objections come up I'll address them in a follow-up, and if there's any major ones we can always revert