Add dateutil to required packages

[bgottula]

dateutil is used in skyplot to parse natural-language dates. It's required by infludb-client, matplotlib, and pandas so it's getting installed by means of those transitive dependencies but since it's used directly it should be included in this package's set of dependencies.

This may not be the only package in this category. Surely there must be tools that do dependency analysis on a Python codebase. Maybe Poetry can do this?