REGR: assure .unique of mixed strings does not stringize
closes #16107

this *might* slow things down (because of the inferring), so will have to make a fix for that.
ok this doesn't regress performance (and helps in a couple of situations). # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16108?src=pr&el=h1) Report
> Merging [#16108](https://codecov.io/gh/pandas-dev/pandas/pull/16108?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f8b25c2829d414f7073ee3a25ba8130e75b4124f?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/16108/graphs/tree.svg?height=150&width=650&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/16108?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16108      +/-   ##
==========================================
+ Coverage   90.83%   90.83%   +<.01%     
==========================================
  Files         159      159              
  Lines       50768    50771       +3     
==========================================
+ Hits        46117    46120       +3     
  Misses       4651     4651
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.61% <100%> (ø)` | :arrow_up: |
| #single | `40.33% <0%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16108?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/algorithms.py](https://codecov.io/gh/pandas-dev/pandas/pull/16108?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYWxnb3JpdGhtcy5weQ==) | `94.44% <100%> (+0.02%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16108?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16108?src=pr&el=footer). Last update [f8b25c2...17cc098](https://codecov.io/gh/pandas-dev/pandas/pull/16108?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
