feat: add "complete" timezone support

at least for now, this is all we need i think..  if nothing
configured, will fallback to default system local timezone.  or can
configure the default, or alternate(s) as needed.

some day when we drop support for python 3.8, can also remove the
python-dateutil dependency..

docs/conf.py

@@ -30,6 +30,7 @@ exclude_patterns = ["_build", "Thumbs.db", ".DS_Store"]
 
 intersphinx_mapping = {
     "alembic": ("https://alembic.sqlalchemy.org/en/latest/", None),
+    "dateutil": ("https://dateutil.readthedocs.io/en/stable/", None),
     "humanize": ("https://humanize.readthedocs.io/en/stable/", None),
     "mako": ("https://docs.makotemplates.org/en/latest/", None),
     "packaging": ("https://packaging.python.org/en/latest/", None),