BUG: Categorical of booleans has object .categories

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Reproducible Example

In [1]: import pandas as pd
   ...: 
   ...: obs = pd.DataFrame({"int_cat":[1,2,3], "bool_cat":[True,False,False]}, dtype="category")
   ...: obs["bool_cat"].cat.categories.dtype
Out[1]: dtype('O')

In [2]: bools = pd.Series([True, False, True])
   ...: bools.dtype
Out[2]: dtype('bool')

In [3]: bools.astype("category").cat.categories.dtype
Out[3]: dtype('O')

In [4]: pd.__version__
Out[4]: '1.4.1'

Issue Description

I would assume a categorical generated from a boolean array would have bool category values, not object.

This is weird, but it’s admittedly a funny use case. Upstream of https://github.com/theislab/anndata/issues/724

Expected Behavior

I would have expected the dtype of the categories to be boolean.

Installed Versions

``` ------------------ commit : 06d230151e6f18fdb8139d09abf539867a8cd481 python : 3.9.10.final.0 python-bits : 64 OS : Darwin OS-release : 20.6.0 Version : Darwin Kernel Version 20.6.0: Wed Nov 10 22:23:07 PST 2021; root:xnu-7195.141.14~1/RELEASE_X86_64 machine : x86_64 processor : i386 byteorder : little LC_ALL : None LANG : None LOCALE : None.UTF-8

pandas : 1.4.1 numpy : 1.21.5 pytz : 2021.3 dateutil : 2.8.2 pip : 22.0.3 setuptools : 60.5.0 Cython : 0.29.25 pytest : 7.0.1 hypothesis : None sphinx : 4.1.2 blosc : None feather : None xlsxwriter : None lxml.etree : None html5lib : None pymysql : None psycopg2 : None jinja2 : 3.0.3 IPython : 8.1.0 pandas_datareader: None bs4 : None bottleneck : None fastparquet : None fsspec : 2022.02.0 gcsfs : None matplotlib : 3.5.1 numba : 0.55.1 numexpr : 2.8.1 odfpy : None openpyxl : 3.0.9 pandas_gbq : None pyarrow : 7.0.0 pyreadstat : None pyxlsb : None s3fs : None scipy : 1.8.0 sqlalchemy : 1.4.31 tables : 3.7.0 tabulate : 0.8.9 xarray : 0.21.1 xlrd : 1.2.0 xlwt : None zstandard : None

</details>

[phofl]

This is fixed on main.

May need tests

[Kyrpel]

Hi my name is matias, im from argentina, im new to open-source community and would like to try it. so tell me if i can help you with this.

Hi Matias. If you find an issue that interests you, just write in the comments “take” and the issue will be assigned to you, so other contributors know about it. If you want to drop the issue just press “unassign me” at the right of the issue description