Add pydantic dep to pyproject file

[BrianJKoopman]

Description

This just adds pydantic to the pyproject file.

Motivation and Context

Missed this in the review of #427.

How Has This Been Tested?

Ran an install via:

python3 -m pip install .
python3 -m pip install -r requirements/testing.txt

Then ran the unit tests locally. This is essentially what the CI that's failing is doing.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[BrianJKoopman]

pre-commit.ci run