chore: move packaging information to `pyproject.toml`

[SauravMaheshkar]

Changes proposed by this PR can be summarized as follows :-

• Delete setup.py in favour of pyproject.toml for packaging.
• Update workflows to build using python -m build and cache pip.

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pyproject.toml is the new standard for packaging python packages, setup.py is now deprecated (first introduced in PEP 518 and later expanded in PEP 517, PEP 621 and PEP 660).

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.

[epwalsh]

Hey @SauravMaheshkar, I can help get this over the finish line if you want.