Restore the Python 3.8 code paths removed upstream in 4.14.0 (commit 7cfb2c0,
"Drop support for Python 3.8", PR python#585), applied as a revert of that commit on
top of the 4.16.0 tag with the conflicts accumulated in 4.14.0-4.16.0 resolved:

- requires-python >= 3.8, 3.8 classifier, py38 tox env, 3.8/pypy3.8 CI matrix
  entries, package description and docs updated to 3.8+
- reinstate the 3.8 implementations of Annotated/_AnnotatedAlias, TypeAlias,
  TypeGuard, TypeIs, TypeForm, Concatenate, Required, NotRequired, ReadOnly,
  Unpack, ParamSpec, get_origin/get_args and the hasattr(typing, ...) probes,
  plus the 3.8 branches of get_type_hints() and evaluate_forward_ref()
- keep the 4.16.0 aliasing mechanism: Annotated is only assigned from typing
  when it exists, and Union is bound unconditionally so that the 3.8 branches
  are visible to ruff's static analysis
- apply the upstream issue python#661 fix to the 3.8 Unpack alias as well (call
  typing._GenericAlias.__getitem__ directly instead of using zero-arg super()
  inside a class body that assigns __class__), so isinstance() results no
  longer depend on whether a profiling function is set
- provide the 3.9+ subscription of arbitrary TypedDict and NamedTuple types on
  3.8 as well. On 3.9+ it comes for free from dict.__class_getitem__ /
  tuple.__class_getitem__, neither of which exists before PEP 585, and upstream
  4.13.x deliberately left it out. Non-generic TypedDict/NamedTuple classes now
  get a __class_getitem__ returning typing._GenericAlias, with the same
  __origin__/__args__/__parameters__ and instantiation behaviour as 3.9+
- tests: exercise the PEP 585-dependent cases through typing aliases instead of
  skipping them (evaluate_forward_ref nested/globals/owner, get_original_bases,
  NamedTuple subscription) and drop the 3.8 assertions that the above makes
  obsolete; unskip test_missing__name__ (the fix is present in the 3.8 releases
  CI installs); drop the Literal[[], []] deduplication assertion that CPython
  3.13.15/3.14.7 made obsolete (same change as upstream 79c903c)
- drop the daily scheduled runs, which serve the upstream repository only:
  remove the "Third-party tests" workflow (a cron running the test suites of
  nine downstream projects) and the daily cron in ci.yml together with its
  create-issue-on-failure job

Verified locally: unittest suite passes on cp38-cp314 (including the
free-threaded builds), ruff clean, tox py38-py314 green.