vobject: add missing pytz and six dependencies

vobject: add missing pytz and six dependencies

vobject 0.9.9 lists both pytz and six as unconditional runtime requirements
in install_requires, but neither was in DEPENDS. Running the change_tz
console script failed with "ModuleNotFoundError: No module named 'pytz'".

six happened to resolve transitively through python3-dateutil even though
vobject/base.py imports it directly, so list it explicitly rather than rely
on another package's dependency.

Extend test.sh to check that both modules import and that vobject.change_tz
loads.

Fixes: https://github.com/openwrt/packages/issues/29992
Signed-off-by: Alexandru Ardelean <redacted>