BUG: dateutil 2.9.x cannot load tzdata on systems without system zoneinfo (z/OS, AIX, some Unix variants)

[dbaranwal06]

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Reproducible Example

pip install python-dateutil==2.9.0 tzdata
from dateutil.tz import gettz
print(gettz("US/Pacific"))  # returns None on z/OS

Issue Description

On platforms that do not provide a system IANA zoneinfo database (e.g., IBM z/OS, some AIX installations, minimal Unix builds), python-dateutil 2.9.x is unable to load any timezones. This breaks timezone-aware features in pandas, numpy, and many other libraries.

The issue appears to be a regression introduced when bundled tzdata was removed in 2.9.0.

Expected Behavior

When the tzdata package is installed (which provides IANA tz files at tzdata/zoneinfo), dateutil should:

• detect tzdata
• automatically use its zoneinfo files
• return a valid tzinfo object

This behavior existed implicitly in dateutil 2.8.2, which bundled tzdata internally.

Installed Versions

Details

INSTALLED VERSIONS ------------------ commit : None python : 3.13.3 python-bits : 64 OS : OS/390 OS-release : 30.00 Version : 05 machine : 3931 processor : byteorder : big LC_ALL : None LANG : C LOCALE : None.None

pandas : 2.3.3
numpy : 2.0.2
pytz : 2025.2
dateutil : 2.9.0.post0
pip : 25.1.1
Cython : 3.1.3
sphinx : None
IPython : None
adbc-driver-postgresql: None
adbc-driver-sqlite : None
bs4 : None
blosc : None
bottleneck : None
dataframe-api-compat : None
fastparquet : None
fsspec : None
html5lib : None
hypothesis : 6.48.1
gcsfs : None
jinja2 : None
lxml.etree : 6.0.1
matplotlib : None
numba : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
psycopg2 : None
pymysql : None
pyarrow : None
pyreadstat : None
pytest : 8.4.1
python-calamine : None
pyxlsb : None
s3fs : None
scipy : None
sqlalchemy : 2.0.44
tables : None
tabulate : None
xarray : None
xlrd : None
xlsxwriter : None
zstandard : None
tzdata : 2025.2
qtpy : None
pyqt5 : None