API: reimplement FixedWindowIndexer.get_window_bounds
- [x] closes #36040
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

This PR replaces [36132](https://github.com/pandas-dev/pandas/pull/36132) following @mroeschke's [36567](https://github.com/pandas-dev/pandas/pull/36567).

Originally, my PR was to fix Issue [36040](https://github.com/pandas-dev/pandas/issues/36040). But it appears that Matthew's PR already fixed it! Nonetheless, I still included two things from my previous PR:

1. I added tests to cover the bug outlined in Issue [36040](https://github.com/pandas-dev/pandas/issues/36040).
2. I reimplemented FixedWindowIndexer.get_window_bounds in a way that I believe if a lot more intuitive and simple. Please lmk if you disagree, in which case I could remove this from the PR.

I don't believe any functionality was altered by this PR so I did not include a whatsnew entry
cc @mroeschke  Could you also add a whatsnew entry in 1.2? Looks like there were some failures, but I do not believe those were caused by this PR > Could you also add a whatsnew entry in 1.2?

@mroeschke I added a whatsnew entry about the reimplementation and linked Issue [36040](https://github.com/pandas-dev/pandas/issues/36040), even though it's not technically fixing that issue (the issue was already fixed by your PR). Please lmk if this is what you intended I add, or if you were looking for something else. thanks @justinessert 