ENH: implement Timedelta.__mod__ and __divmod__
This implements exclusively the `__mod__, __rmod__, __divmod__, __rdivmod__` parts of #19365, does _not_ implement any of the bugfixes or tests that got rolled into that PR.  Several tests are xfailed for now, will be fixed after #19738.  These tests will be rebased/moved to test_arithmetic after #19752.

- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19755?src=pr&el=h1) Report
> Merging [#19755](https://codecov.io/gh/pandas-dev/pandas/pull/19755?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/a76e5b48c56e5429377aee07e8d7119d264fdbae?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19755/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/19755?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #19755   +/-   ##
=======================================
  Coverage    91.6%    91.6%           
=======================================
  Files         150      150           
  Lines       48864    48864           
=======================================
  Hits        44763    44763           
  Misses       4101     4101
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.98% <ø> (ø)` | :arrow_up: |
| #single | `41.77% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19755?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19755?src=pr&el=footer). Last update [a76e5b4...aefa6d6](https://codecov.io/gh/pandas-dev/pandas/pull/19755?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 lgtm ping on green. Ping thanks!