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.
```

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21887?src=pr&el=h1) Report
> Merging [#21887](https://codecov.io/gh/pandas-dev/pandas/pull/21887?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/bdb61688aa80a6ec9bdb6832946dd83ab4b4c9c8?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21887/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/21887?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21887      +/-   ##
==========================================
- Coverage   91.91%   91.91%   -0.01%     
==========================================
  Files         164      164              
  Lines       49992    49988       -4     
==========================================
- Hits        45952    45945       -7     
- Misses       4040     4043       +3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.29% <100%> (-0.01%)` | :arrow_down: |
| #single | `42.17% <77.19%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21887?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/21887/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.53% <ø> (-0.05%)` | :arrow_down: |
| [pandas/core/strings.py](https://codecov.io/gh/pandas-dev/pandas/pull/21887/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3RyaW5ncy5weQ==) | `98.63% <ø> (ø)` | :arrow_up: |
| [pandas/core/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/21887/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYmFzZS5weQ==) | `96.92% <ø> (ø)` | :arrow_up: |
| [pandas/core/indexes/multi.py](https://codecov.io/gh/pandas-dev/pandas/pull/21887/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9tdWx0aS5weQ==) | `95.12% <100%> (+0.08%)` | :arrow_up: |
| [pandas/core/accessor.py](https://codecov.io/gh/pandas-dev/pandas/pull/21887/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYWNjZXNzb3IucHk=) | `98.71% <100%> (ø)` | :arrow_up: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/21887/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `94.11% <100%> (ø)` | :arrow_up: |
| [pandas/core/algorithms.py](https://codecov.io/gh/pandas-dev/pandas/pull/21887/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYWxnb3JpdGhtcy5weQ==) | `94.56% <100%> (-0.29%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/21887/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `85.1% <100%> (-0.23%)` | :arrow_down: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/21887/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `92.11% <100%> (-0.09%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/21887/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.19% <100%> (ø)` | :arrow_up: |
| ... and [4 more](https://codecov.io/gh/pandas-dev/pandas/pull/21887/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21887?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/21887?src=pr&el=footer). Last update [bdb6168...62e2b2a](https://codecov.io/gh/pandas-dev/pandas/pull/21887?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Rebasing issues - will fix and update later - only 3 files were changed @KalyanGokhale : In general for this repository, you don't need to close the PR if there are rebasing issues.  You can fix them here if you would like. Thanks @gfyoung I think I am finding my way around git :), had tried to reopen this PR earlier but GitHub won't allow as I did force push.....will drop a note if I need any pointers, just trying to figure out a few things