gh-122575: gh-142349: fix sys.flags tuple size (it unintentionally increased)

[gpshead]

See #122575 (comment) for rationale. the lazy imports PEP initial implementation inadvertently increased the length of the sys.flags tuple. In a way that did not do anything useful or related to lazy imports (it exposed sys.flags.gil in the tuple). This fixes that to hard code the length to the 3.13 length of 18 and have our tests and code comments make it clear that we've since stopped making new sys.flags attributes available via sequence index.

internal implementation detail, no news entry. pulls an equivalent change to do this out of already large PR #129337 .

• Issue: sys.flags.gil should be a "sequence" attribute OR stop adding to the sequence #122575

• Issue: Implement PEP 810 – Explicit lazy imports #142349