PypiVersionAPI.get_latest_date() crashes when a release entry is missing upload_time_iso_8601

[drawliin]

Summary

PypiVersionAPI.get_latest_date() in packagedb/package_managers.py can fail when a PyPI release entry is missing upload_time_iso_8601.

The current implementation only assigns current_date when that field is present, but still uses current_date later in the loop even when it was never set. That can raise UnboundLocalError instead of skipping the malformed entry.

Current behavior

A case like this can fail:

downloads = [
    {},
    {"upload_time_iso_8601": "2010-12-23T05:14:23.509436Z"},
]