Make dependency on python-dateutil explicit

[mfisher87]

Our source code depends on dateutil. Conda Forge's autotick bot recognizes this and complains that the dependency isn't explicitly specified:

Packages found by source code inspection but not in the meta.yaml:

• python-dateutil

conda-forge/earthdata-feedstock#35

This dependency is present implicitly through another dependency, but since our source depends on it directly, I feel it should be explicitly named in our specification (both pyproject.toml and feedstock meta.yaml)