[3.15] gh-150700: Fix class-scope inline comprehensions when nested scopes reference...

[3.15] gh-150700: Fix class-scope inline comprehensions when nested scopes reference `__class__` and friends (GH-150735) (#151211)

gh-150700: Fix class-scope inline comprehensions when nested scopes reference `__class__` and friends (GH-150735)

* Fix class-scope inline comprehensions when nested scopes reference `__class__` and friends

In `inline_comprehension()`, when `__class__` / `__classdict__` /
`__conditional_annotations__` appears as `FREE` in a comprehension's
symbol table because a nested scope captured it (e.g. nested lambdas),
this name is still discarded from `comp_free` unconditionally.

This prevents `drop_class_free()` from seeing it, so the appropriate
`ste_needs_(...)` flag is never set on the enclosing class.
That leads to `codegen_make_closure()` throwing `SystemError` when it
couldn't find `__class__` / `__classdict__` /
`__conditional_annotations__` in the class's cellvars.

From now on we just discard from `comp_free` when no child scope
(e.g. a lambda) still needs the name as `FREE`. When a child scope does
need it, keep it in `comp_free` so `drop_class_free()` can set the
appropriate flag and the class creates the implicit cell.

* Fix tests

* Fix typo

* Fix formatting

* Add test checking validity of `__class__` returned

* Prefer 'used' to 'deferred'
(cherry picked from commit ce916dc50644bb1de940f5fb580bd9907cceb959)

Co-authored-by: Bartosz Sławecki <bartosz@ilikepython.com>