gh-125900: Clean-up logic around immortalization in free-threading

[colesbury]

• Remove @suppress_immortalization decorator
• Make suppression flag per-thread instead of per-interpreter
• Suppress immortalization in eval() to avoid refleaks in three tests (test_datetime.test_roundtrip, test_logging.test_config8_ok, and test_random.test_after_fork).
• frozenset() is constant, but not a singleton. When run multiple times, the test could fail due to constant interning.

• Issue: gh-125900