BUG: zero-pad shorter years in `Timestamp.isoformat`
As discussed in #50867, this changes `Timestamp.isoformat` to zero-pad years to 4 digits.

After applying the change, I also had to change a existing test because of the now-changed standard format, but also remove a condition which assumed that `"001-01-01"` and `"0001-01-01"` would result in different objects (not sure, maybe there was a time where the resulting `Timestamp` object would not be equal?). However, on current `main`:
```python
a = pd.Timestamp("001-01-01")
b = pd.Timestamp("0001-01-01")
a == b and a.unit == b.unit
```

cc @spencerkclark

No `whatsnew` entry yet because I was not sure where to put it.

- [x] closes #50867
- [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.~
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

I wonder if it would be possible to include this in `pandas` 2.0? The actual change is very small and it fixes a bug in one of the new features of 2.0, but of course we're pretty late in the release process already. Thanks @keewis 