Move pytest.ini into pyproject.toml

[romanc]

Description

This PR moves the contents of pytest.ini into pyproject.toml where all project configuration can be centralized. This is a follow-up from #39, which introduced pyproject.toml in the first place.

How Has This Been Tested?

CI will complain if something were not okay anymore.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation: N/A
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules: N/A
• New check tests, if applicable, are included: N/A