Update dependencies

[jhkennedy]

When releasing the last version of earthdata on conda-forge in conda-forge/earthdata-feedstock#11, the automatic dependency analysis picked up on some out of date dependencies:

Packages found by source code inspection but not in the meta.yaml:
    python-dateutil
Packages found in the meta.yaml but not found by source code inspection:
    pydantic

This:

• Adds python-dateutil as it is directly used here: https://github.com/nsidc/earthdata/blob/main/earthdata/search.py#L4
• Removes pydantic as is appears unused; I removed it both as a dependency and as mentioned in the README.md
• updates the poetry.lock file accordingly

I also fixed a minor typo in the contributing guide

[betolink]

Good PR!