CI: add macOS and Python 3.7–3.11 matrix, update actions and pip handling

[uddaloksarkar]

Motivation

• Expand the GitHub Actions CI matrix to run on macOS in addition to Ubuntu and to test multiple Python versions for broader compatibility.
• Update outdated GitHub Action usages to newer, maintained versions and the OCaml setup action for better compatibility and security.
• Ensure pip/setuptools upgrades are compatible with older Python (3.7) by applying version constraints when necessary.

Description

• Added macos-latest to the matrix.os and introduced a matrix.python with 3.7–3.11 to the CI matrix.
• Upgraded actions/checkout@v1 to actions/checkout@v4, avsm/setup-ocaml@v1 to ocaml/setup-ocaml@v3, and actions/setup-python@v1 to actions/setup-python@v5.
• Switched the Python setup to use the matrix value and added a conditional pip/setuptools upgrade that pins compatible versions for Python 3.7.
• Kept gelpia build and tests scoped to Ubuntu and preserved existing build/test steps (make, make test, and the gelpia-backed test variant).

Testing

• No automated tests were executed locally as part of this change.
• The CI workflow will run make to build and make test to run the test suite on each matrix entry and will run the gelpia-backed tests on Ubuntu.
• The workflow changes are limited to the Actions configuration and are intended to be validated by GitHub Actions on push.

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Codex Task