BUG: merging with a boolean/int categorical column
Additional check prevents trying to change types of categorical blocks.

- [x] closes #17187
- [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/17841?src=pr&el=h1) Report
> Merging [#17841](https://codecov.io/gh/pandas-dev/pandas/pull/17841?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/727ea20bdb72ff4a5b8380d7f613d4aff058e28a?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/17841/graphs/tree.svg?src=pr&width=650&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/17841?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17841      +/-   ##
==========================================
- Coverage   91.22%   91.21%   -0.02%     
==========================================
  Files         163      163              
  Lines       50014    50014              
==========================================
- Hits        45627    45618       -9     
- Misses       4387     4396       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.01% <100%> (ø)` | :arrow_up: |
| #single | `40.24% <100%> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17841?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/internals.py](https://codecov.io/gh/pandas-dev/pandas/pull/17841?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzLnB5) | `94.38% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17841?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17841?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.77% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17841?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/17841?src=pr&el=footer). Last update [727ea20...95474f8](https://codecov.io/gh/pandas-dev/pandas/pull/17841?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17841?src=pr&el=h1) Report
> Merging [#17841](https://codecov.io/gh/pandas-dev/pandas/pull/17841?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/727ea20bdb72ff4a5b8380d7f613d4aff058e28a?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/17841/graphs/tree.svg?height=150&width=650&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/17841?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17841      +/-   ##
==========================================
- Coverage   91.22%   91.21%   -0.01%     
==========================================
  Files         163      163              
  Lines       50014    50075      +61     
==========================================
+ Hits        45627    45678      +51     
- Misses       4387     4397      +10
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.02% <100%> (+0.01%)` | :arrow_up: |
| #single | `40.29% <100%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17841?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/internals.py](https://codecov.io/gh/pandas-dev/pandas/pull/17841?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzLnB5) | `94.45% <100%> (+0.06%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17841?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/compat/numpy/function.py](https://codecov.io/gh/pandas-dev/pandas/pull/17841?src=pr&el=tree#diff-cGFuZGFzL2NvbXBhdC9udW1weS9mdW5jdGlvbi5weQ==) | `92.12% <0%> (-1.22%)` | :arrow_down: |
| [pandas/core/indexing.py](https://codecov.io/gh/pandas-dev/pandas/pull/17841?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhpbmcucHk=) | `92.82% <0%> (-0.19%)` | :arrow_down: |
| [pandas/io/formats/format.py](https://codecov.io/gh/pandas-dev/pandas/pull/17841?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvZm9ybWF0LnB5) | `95.94% <0%> (-0.13%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17841?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.75% <0%> (-0.12%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/17841?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.48% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/computation/align.py](https://codecov.io/gh/pandas-dev/pandas/pull/17841?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tcHV0YXRpb24vYWxpZ24ucHk=) | `97.89% <0%> (-0.05%)` | :arrow_down: |
| [pandas/core/reshape/concat.py](https://codecov.io/gh/pandas-dev/pandas/pull/17841?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9jb25jYXQucHk=) | `97.57% <0%> (-0.04%)` | :arrow_down: |
| [pandas/core/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/17841?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS5weQ==) | `91.98% <0%> (-0.02%)` | :arrow_down: |
| ... and [17 more](https://codecov.io/gh/pandas-dev/pandas/pull/17841?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17841?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/17841?src=pr&el=footer). Last update [727ea20...6f5b637](https://codecov.io/gh/pandas-dev/pandas/pull/17841?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @jdrudolph nice patch! keep em coming