ci/cd: modernize release pipeline and switch project versioning to setuptools_scm

[mxamin]

• Reworked CD workflow in .github/workflows/cd.yml:

  • Added workflow concurrency controls and default least-privilege permissions.
  • Added a dedicated sdist build job using Python 3.14 with memcached service validation.
  • Added dynamic wheel matrix generation using cibuildwheel build identifiers.
  • Switched wheel builds to pypa/cibuildwheel action and per-target artifact naming.
  • Updated checkout/setup actions and Linux QEMU setup for broader architecture support.
  • Ran tests using generated wheel artificat
  • Reworked release job to download merged wheel/sdist artifacts and publish draft GitHub releases.
  • Removed legacy artifact zip packaging flow and old action versions.

• Updated CI workflow in .github/workflows/ci.yml:

  • Normalized trigger syntax for push and pull_request events.
  • Extended test matrix with Python 3.14.
  • Upgraded checkout/setup-python action versions and enabled tag fetching.
  • Removed obsolete LIBMEMCACHED_VERSION environment usage.

• Introduced pyproject.toml build configuration:

  • Added setuptools build-system metadata with setuptools_scm integration.
  • Configured setuptools_scm to write src/pylibmc/_version.py and disable local version suffixes.
  • Added cibuildwheel global config, per-platform arch settings, and platform-specific dependency install overrides.

• Updated packaging/runtime integration:

  • setup.py now uses SCM-derived versions (use_scm_version + setup_requires).
  • src/pylibmc/init.py now prefers generated package version with extension version fallback.
  • Added setuptools_scm to requirements_test.txt.
  • Added src/pylibmc/_version.py to .gitignore.
  • Made bin/runtests.py shebang interpreter-agnostic by using "python" instead of "python3".

[mxamin]

@lericson Can you take a look when you have time?