BUG: Fixes unwanted casting in .isin (GH21804)
- [x] closes #21804 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Results from running asvs on algorithms are:
```
       before           after         ratio
     [365eac4d]       [ee66578f]
+        1.91±0ms      2.28±0.02ms     1.20  algorithms.Hashing.time_series_timedeltas
-     2.34±0.02ms         1.88±0ms     0.80  algorithms.Hashing.time_series_int

SOME BENCHMARKS HAVE CHANGED SIGNIFICANTLY.
```