Fix PyPI latest-date handling when upload_time_iso_8601 is missing

[drawliin]

Closes: #863

Summary

Fix PypiVersionAPI.get_latest_date() so PyPI release entries without
upload_time_iso_8601 are skipped instead of causing an UnboundLocalError.

Changes

• skip invalid download entries that do not include upload_time_iso_8601
• keep returning the latest valid parsed timestamp
• return None when no valid timestamps are available
• add regression tests for mixed valid/invalid entries
• add regression tests for the all-invalid case

Why

The previous implementation could reference current_date before assignment
when a PyPI download entry was missing upload_time_iso_8601.

Testing

I added coverage in packagedb/tests/test_package_managers.py for:

• entries with missing timestamps followed by valid ones
• entries with no valid timestamps