BUG: Keep float dtype in merge on int and float column
- [ ] closes #16572
- [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/18352?src=pr&el=h1) Report
> Merging [#18352](https://codecov.io/gh/pandas-dev/pandas/pull/18352?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/fedc503f0d25b04850a931398fc0556d892d69ea?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18352/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/18352?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18352      +/-   ##
==========================================
- Coverage   91.35%   91.34%   -0.02%     
==========================================
  Files         163      163              
  Lines       49691    49697       +6     
==========================================
- Hits        45397    45394       -3     
- Misses       4294     4303       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.14% <100%> (ø)` | :arrow_up: |
| #single | `39.67% <0%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18352?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/reshape/merge.py](https://codecov.io/gh/pandas-dev/pandas/pull/18352?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9tZXJnZS5weQ==) | `94.32% <100%> (+0.05%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18352?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18352?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.8% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18352?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/18352?src=pr&el=footer). Last update [fedc503...a3d6fe6](https://codecov.io/gh/pandas-dev/pandas/pull/18352?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback thanks for the comments. Green now. thanks @reidy-p nice patch and great responsiveness!