remove unreachable states, assert they are unreachable just in case
No luck so far in tracking down the issue where this was discussed.

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

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19180?src=pr&el=h1) Report
> Merging [#19180](https://codecov.io/gh/pandas-dev/pandas/pull/19180?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/8acdf801c501a0ce2ac14ddd676cd248f0f32180?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19180/graphs/tree.svg?width=650&height=150&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/19180?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19180      +/-   ##
==========================================
+ Coverage   91.52%   91.52%   +<.01%     
==========================================
  Files         147      148       +1     
  Lines       48775    48776       +1     
==========================================
+ Hits        44639    44642       +3     
+ Misses       4136     4134       -2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.89% <100%> (ø)` | :arrow_up: |
| #single | `41.61% <100%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19180?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/internals.py](https://codecov.io/gh/pandas-dev/pandas/pull/19180/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzLnB5) | `94.49% <100%> (+0.06%)` | :arrow_up: |
| [pandas/core/dtypes/cast.py](https://codecov.io/gh/pandas-dev/pandas/pull/19180/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2Nhc3QucHk=) | `88.42% <0%> (-0.16%)` | :arrow_down: |
| [pandas/tools/hashing.py](https://codecov.io/gh/pandas-dev/pandas/pull/19180/diff?src=pr&el=tree#diff-cGFuZGFzL3Rvb2xzL2hhc2hpbmcucHk=) | `100% <0%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19180?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/19180?src=pr&el=footer). Last update [8acdf80...d27bcc5](https://codecov.io/gh/pandas-dev/pandas/pull/19180?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 CI has been on the fritz for a few days.  Should I keep trying or wait until further notice? this is fine. generally if the CI fails for an othogonal reason to the PR don't keep pushing.