refactor: migrate from github3.py to PyGithub

[jmeridth]

Proposed Changes

Replace github3.py with PyGithub to align with the other OSPO repos (cleanowners, measure-innersource, pr-conflict-detector, evergreen) that have already completed this migration.

What changed

• auth.py: Rewrote authentication from github3.login/login_as_app_installation to PyGithub's Auth.Token/Auth.AppAuth + get_installation_auth
• stale_repos.py: Updated all API calls (organization() -> get_organization(), repositories() -> get_repos(), topics().names -> get_topics(), etc.), exception types (NotFoundError -> UnknownObjectException, GitHubException -> GithubException), and dropped dateutil since PyGithub returns datetime objects directly
• test_auth.py / test_stale_repos.py: Updated mock targets, exception constructors, and pushed_at from ISO strings to datetime objects
• pyproject.toml: Swapped github3-py for PyGithub>=2.6.0, dropped python-dateutil, types-python-dateutil, and types-requests
• Linter configs: Updated .mypy.ini and .isort.cfg to reference github instead of github3

Dependencies removed

• github3-py
• python-dateutil
• types-python-dateutil (dev)
• types-requests (dev)

Dependencies added

• PyGithub>=2.6.0

Readiness Checklist

Author/Contributor

• If documentation is needed for this change, has that been included in this pull request
• run make lint and fix any issues that you have introduced
• run make test and ensure you have test coverage for the lines you are introducing

Proof it works

53 tests pass with 100% coverage. All pre-commit hooks pass.

Risk + AI role

Low -- direct library swap with identical behavior, no new features.

Review focus

• Auth patterns in auth.py (AppAuth/Token vs login_as_app_installation)
• datetime handling in stale_repos.py (PyGithub returns datetime objects directly, so dateutil was dropped)

[Copilot]

Pull request overview

This PR migrates the action’s GitHub API integration from github3.py to PyGithub, updating authentication, repository queries, and exception handling to match the newer client library while keeping existing behavior and test coverage.

Changes:

• Replaced github3.py API usage with PyGithub equivalents across auth + stale-repo discovery logic.
• Updated unit tests and mocks to reflect PyGithub return types (notably datetime) and method names.
• Updated dependencies/lockfile and type-checker/import-sorting configs for the new package.

Reviewed changes

Copilot reviewed 7 out of 10 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
auth.py	Swaps auth implementation to PyGithub (Auth.Token / Auth.AppAuth + installation auth).
stale_repos.py	Updates org/user repo listing, topics, releases/PRs, and exception handling for PyGithub.
test_auth.py	Adjusts patches and assertions for the new auth flow and constructors.
test_stale_repos.py	Updates mocks to PyGithub method names/iterables and datetime-based fields.
pyproject.toml	Replaces github3-py with PyGithub>=2.6.0 and removes dateutil-related deps.
uv.lock	Reflects dependency graph changes for PyGithub and removed packages.
.github/linters/.mypy.ini	Updates ignore section from github3.* to github.*.
.github/linters/.isort.cfg	Updates import categorization for the new module names.
.pylintrc	Minor formatting tweak in linter config.
.coveragerc	Minor formatting tweak in coverage config.

[zkoppert]

Two small follow-ups plus a consistency thought on the now-dead auth guard. Details inline.

[zkoppert]

🚀