[3.12] gh-109118: Disallow nested scopes within PEP 695 scopes within classes (GH-109196) (#109297)

gh-109118: Disallow nested scopes within PEP 695 scopes within classes (GH-109196)

Fixes GH-109118. Fixes GH-109194.

(cherry picked from commit b88d9e75f68f102aca45fa62e2b0e2e2ff46d810)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
Co-authored-by: Carl Meyer <carl@oddbird.net>