chore: move configurations to `pyproject.toml`

[SauravMaheshkar]

This PR aims to minimize the number of configuration files for the project. Instead of a separate pytest.ini and mypy.ini, we now would have all the configurations within the pyproject.toml file leading to a overall minimal code structure.

Before submitting

• [x] I've read and followed all steps in the Making a pull request section of the CONTRIBUTING docs.
• [x] I've updated or added any relevant docstrings following the syntax described in the Writing docstrings section of the CONTRIBUTING docs.
• [x] If this PR fixes a bug, I've added a test that will fail without my fix.
• [x] If this PR adds a new feature, I've added tests that sufficiently cover my new functionality.

[SauravMaheshkar]

More than happy to make upstream PRs to all the other packages within the allenai organization that were created using this template.

[SauravMaheshkar]

This is great! Thank you @SauravMaheshkar!

Feel free to make PRs in other repos if you have time. You can ping me for a review.

Yeah I'll make follow up PRs to other repositories.