BUG: Fixes unwanted casting in .isin (GH21804)
- [x] closes #xxxx
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

ASV output for algorithms is as follows:
```python
       before           after         ratio
     [5380fcd4]       [99ac16f4]
+     1.93±0.01ms      2.38±0.01ms     1.23  algorithms.Hashing.time_series_float

SOME BENCHMARKS HAVE CHANGED SIGNIFICANTLY.
```
