Use borrowed pointers in TYPE_CACHE instead of strong references by youknowone · Pull Request #7384 · RustPython/RustPython

…tPython#7384)

The method cache (TYPE_CACHE) was storing strong references (Arc
clones) to cached attribute values, which inflated sys.getrefcount().
This caused intermittent test_memoryview failures where refcount
assertions would fail depending on GC collection timing.

Store borrowed raw pointers instead. Safety is guaranteed because:
- type_cache_clear() nullifies all entries during GC collection,
  before the collector breaks cycles
- type_cache_clear_version() nullifies entries when a type is
  modified, before the source dict entry is removed
- Readers use try_to_owned_from_ptr (safe_inc) to atomically
  validate and increment the refcount on cache hit