ci: add checks for Python syntax (to catch any usage of unsupported language features)

[DavidVujic]

Description

Set the Python version explicitly in the CI config.
Run mypy to verify the Python syntax - to avoid introducing something not working in Python 3.8.

Motivation and Context

Fixes #30

How Has This Been Tested?

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:

• I have read the code of conduct.
• I have read the contributing guide.
• I have updated the documentation accordingly (if applicable).