Add pytest coverage

[Will-Tyler]

Overview

This pull request adds pytest coverage to the vcztools project. I find the coverage information useful and think others might benefit from it as well.

This pull request makes progress on #56. What remains is to add the Coveralls configuration.

Testing

I tested these changes manually by installing the dev requirements (`pip install -e ".[dev]") and then running the tests ('pytest tests'). These commands produce the desired coverage file.

References

• pytest project.toml configuration
• pytest-cov configuration