fix hashing string-casting error
- [x] closes #21002
- [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/21187?src=pr&el=h1) Report
> Merging [#21187](https://codecov.io/gh/pandas-dev/pandas/pull/21187?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/b36b451a74bc16d7ea64c158a3cd33fbfb504068?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `11.11%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21187/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/21187?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21187      +/-   ##
==========================================
- Coverage   91.92%    91.9%   -0.02%     
==========================================
  Files         153      153              
  Lines       49563    49572       +9     
==========================================
+ Hits        45559    45560       +1     
- Misses       4004     4012       +8
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.3% <11.11%> (-0.02%)` | :arrow_down: |
| #single | `41.8% <11.11%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21187?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/21187/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `85.27% <11.11%> (-0.7%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21187?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/21187?src=pr&el=footer). Last update [b36b451...7ab9324](https://codecov.io/gh/pandas-dev/pandas/pull/21187?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!

and soon enough we will blow away all PY2 code in any event! but nice patch.