Modern packaging and deterministic CI

[waddell]

Objective

Establish a modern packaging and deterministic CI baseline for UrbanAccess before recovering the larger feature set in PR #94.

Tracked under #95. PR #94 remains the source and provenance reference for the broader recovery effort.

Scope

• Establish and document the supported Python versions and a known-working dependency envelope.
• Remove Python 2 classifiers, declarations, and compatibility expectations.
• Add a package-build check.
• Test the installed package rather than only the source checkout.
• Separate deterministic required tests from tests that depend on external network services.
• Add dependency caching where it improves CI runtime without weakening reproducibility.

Acceptance criteria

• CI runs on the supported Python-version matrix.
• The source distribution and wheel build successfully.
• The built artifact installs into a clean environment and passes an import or smoke test.
• Deterministic tests run as required checks without relying on live external services.
• Network-dependent tests are separately marked and documented.
• Package metadata no longer claims Python 2 support.
• The PR documents the tested dependency envelope and any remaining failures.
• Contributor provenance from compat for new pandas #94 is preserved or credited wherever work is recovered from it.

Pull request guidance

Open a focused PR against main. Reference #95 and #94 in the PR body without using a closing keyword; these issues cover work beyond this component.