BUG: Timezones are not preserved in Series.mode() output

[willsthompson]

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• (optional) I have confirmed this bug exists on the master branch of pandas.

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Code Sample, a copy-pastable example

import datetime
times = pd.Series([pd.Timestamp(datetime.datetime.now(), tz='US/Central') for n in range(5)])
times.dtype # datetime64[ns, US/Central] == pd.DatetimeTZDtype(tz='US/Central')
mode_result = times.mode()
mode_result.dtype # dtype('<M8[ns]') == np.dtype('datetime64[ns]')

Problem description

The Timestamps are normalized to (I think) UTC time, and the original timezone and offset are lost. Series.median() does preserve timezones, so I assumed this is just an unhandled edge case. I'm working around this using scipy.stats.mode, which does preserve timezones.

Expected Output

times.mode().dtype == pd.DatetimeTZDtype(tz='US/Central')

Output of pd.show_versions()

INSTALLED VERSIONS

commit : 2cb9652
python : 3.8.7.final.0
python-bits : 64
OS : Darwin
OS-release : 20.5.0
Version : Darwin Kernel Version 20.5.0: Sat May 8 05:10:33 PDT 2021; root:xnu-7195.121.3~9/RELEASE_X86_64
machine : x86_64
processor : i386
byteorder : little
LC_ALL : None
LANG : None
LOCALE : None.UTF-8
pandas : 1.2.4
numpy : 1.20.2
pytz : 2021.1
dateutil : 2.8.1
pip : 20.3.4
setuptools : 49.2.1
Cython : 0.29.14
pytest : 6.2.4
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : 2.8.6 (dt dec pq3 ext lo64)
jinja2 : 2.11.3
IPython : 7.24.1
pandas_datareader: None
bs4 : None
bottleneck : None
fsspec : None
fastparquet : None
gcsfs : None
matplotlib : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : 2.0.0
pyxlsb : None
s3fs : None
scipy : 1.6.1
sqlalchemy : 1.3.23
tables : None
tabulate : 0.8.9
xarray : None
xlrd : None
xlwt : None
numba : None

[yasuomaidana]

Could you assign it to me?

[simonjayhawkins]

from https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#where-to-start

So, a good way to find an issue to start contributing to pandas is to check the list of unassigned good first issues and assign yourself one you like by writing a comment with the exact text take.

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